The contractual obligations of the parties may be express or implied.



This provision restates the widely accepted principle according to which the obligations of the parties are not necessarily limited to that which has been expressly stipulated in the contract. Other obligations may be implicit (see Article 5.1.2, Comments and Illustrations).

Close links exist between this Article and some of the other provisions of the Principles. Thus Article 5.1.1 is a direct corollary of the rule according to which “[e]ach party must act in accordance with good faith and fair dealing in international trade” (Article 1.7). Insofar as the rules on interpretation (Chapter 4) provide criteria for filling lacunae (besides criteria for solving ambiguities), those rules may assist in determining the precise content of the contract and therefore in establishing the terms which must be considered as implied.

Implied obligations stem from 


(a) the nature and purpose of the contract;

(b) practices established between the parties and usages;

(c) good faith and fair dealing;

(d) reasonableness.



This Article describes the sources of implied obligations. Different reasons may account for the fact that they have not been expressly stated. The implied obligations may for example have been so obvious, given the nature or the purpose of the obligation, that the parties felt that the obligations “went without saying”. Alternatively, they may already have been included in the practices established between the parties or prescribed by trade usages according to Article 1.9. Yet again, they may be a consequence of the principles of good faith and fair dealing and reasonableness in contractual relations.



1. A rents a full computer network to B and installs it. The contract says nothing as to A’s possible obligation to give B at least some basic information concerning the operation of the system. This may however be considered to be an implied obligation since it is obvious, and necessary for the accomplishment of the purpose of such a contract, that the provider of sophisticated goods should supply the other party with a minimum of information (see Article 5.1.2(a)).

2. A broker who has negotiated a charter party claims the commission due. Although the brokerage contract is silent as to the time when the commission is due, the usages of the sector can provide an implied term according to which the commission is due, for example, only when the hire is earned, or alternatively when the charter party was signed, regardless of whether or not the hire will effectively be paid (see Article 5.1.2(b)).

3. A and B, who have entered into the negotiation of a co-operation agreement, conclude an agreement concerning a complex feasibility study, which will be most time-consuming for A. Long before the study is completed, B decides that it will not pursue the negotiation of the co-operation agreement. Even though nothing has been stipulated regarding such a situation, good faith requires B to notify A of its decision without delay (see Article 5.1.2(c)).

Each party shall co-operate with the other party when such co-operation may reasonably be expected for the performance of that party’s obligations.



A contract is not merely a meeting point for conflicting interests but must also, to a certain extent, be viewed as a common project in which each party must cooperate. This view is clearly related to the principle of good faith and fair dealing (see Article 1.7) which permeates the law of contract, as well as to the obligation to mitigate harm in the event of non-performance (see Article 7.4.8).

The duty of co-operation must of course be confined within certain limits (the provision refers to reasonable expectations), so as not to upset the allocation of duties in the performance of the contract. Although the principal concern of the provision is the duty not to hinder the other party’s performance, there may also be circumstances which call for more active co-operation.



1. A, after contracting with B for the immediate delivery of a certain quantity of oil, buys all the available oil on the spot market from another source. Such conduct, which will hinder B in performing its obligation, is contrary to the duty of co-operation.

2. A, an art gallery in country X, buys a sixteenth century painting from B, a private collector in country Y. The painting may not be exported without a special authorisation and the contract requires B to apply for that permission. B, who has no experience of such formalities, encounters serious difficulties with the application whereas A is familiar with such procedures. In these circumstances, and notwithstanding the contractual provision, A can be expected to give at least some assistance to B.

(1) To the extent that an obligation of a party involves a duty to achieve a specific result, that party is bound to achieve that result. 

(2) To the extent that an obligation of a party involves a duty of best efforts in the performance of an activity, that party is bound to make such efforts as would be made by a reasonable person of the same kind in the same circumstances.



1. Distinction between the duty to achieve a specific result and the duty of best efforts

The degree of diligence required of a party in the performance of an obligation varies considerably depending upon the nature of the obligation incurred. Sometimes a party is bound only by a duty of best efforts. That party must then exert the efforts that a reasonable person of the same kind would exert in the same circumstances, but does not guarantee the achievement of a specific result. In other cases, however, the obligation is more onerous and such a specific result is promised.

The distinction between a “duty to achieve a specific result” and a “duty of best efforts” corresponds to two frequent and typical degrees of severity in the assumption of a contractual obligation, although it does not encompass all possible situations.

Obligations of both types may coexist in the same contract. For instance, a firm that repairs a defective machine may be considered to be under a duty of best efforts concerning the quality of the repair work in general, and under a duty to achieve a specific result as regards the replacement of certain spare parts.


2. Distinction provides criteria for determining whether a party has performed its obligations

Taken together, the two paragraphs of this Article provide judges and arbitrators with criteria by which correct performance can be evaluated. In the case of an obligation to achieve a specific result, a party is bound simply to achieve the promised result, failure to achieve which amounts in itself to non-performance, subject to the application of the force majeure provision (see Article 7.1.7). On the other hand, the assessment of non-performance of an obligation of best efforts calls for a less severe judgment, based on a comparison with the efforts a reasonable person of the same kind would have made in similar circumstances. This distinction signifies that more will be expected from a highly specialised firm selected for its expertise than from a less sophisticated partner.



1. A, a distributor, promises that it will reach a quota of 15,000 sales within a year in the contract zone. If at the end of the period A has sold only 13,000 items, it has clearly failed to perform its obligation (see Article 5.1.4(1)).

2. B, another distributor, promises “to use our best efforts to expand the sales of the product” in the contract zone, without any stipulation that it must reach a minimum quantity. This provision creates an obligation of best efforts; it obliges B to take all the steps that a reasonable person, placed in similar circumstances (nature of the product, characteristics of the market, importance and experience of the firm, presence of competitors, etc.) would take to promote the sales (advertising, visits to customers, proper service, etc.). B does not promise the specific result of selling a certain number of items per year, but does undertake to do all that can be expected of it when acting as a reasonable person (see Article 5.1.4(2)).

In determining the extent to which an obligation of a party involves a duty of best efforts in the performance of an activity or a duty to achieve a specific result, regard shall be had, among other factors, to 


(a) the way in which the obligation is expressed in the contract;

(b) the contractual price and other terms of the contract;

(c) the degree of risk normally involved in achieving the expected result;

(d) the ability of the other party to influence the performance of the obligation.




1. Criteria for determining the nature of the obligation

It is important to determine whether an obligation involves a duty to achieve a specific result or simply a duty of best efforts, as the obligation is more onerous in the former case. Such a determination may sometimes be difficult. This Article therefore establishes criteria which may offer guidance to parties, judges and arbitrators, although the list is not exhaustive. The problems involved are frequently matters of interpretation.


2. Nature of the obligation as expressed by the contract

The way in which an obligation is expressed in the contract may often be of assistance in determining whether the parties intended to create a duty to achieve a specific result or a duty of best efforts.



1. A, a contractor, agrees to build storage facilities for B, who is most keen that the work be finished in an unusually short time. If A undertakes that “the work will be completed before 31 December”, it assumes an obligation to achieve the specific result of meeting that deadline. If it merely undertakes “to try to complete the work before 31 December”, its obligation involves a duty of best efforts to attempt to meet the deadline, but no guarantee that it will definitely be met (see Article 5.1.5(a)).


3. Price or other terms of the contract

The contractual price or other terms of the contract may also offer clues as to the nature of an obligation. An unusually high price or another particular non-monetary reciprocal obligation may indicate a duty to achieve a specific result in cases where a mere duty of best efforts would normally be assumed. Clauses linking payment of the price to the successful outcome of the operation, penalty clauses applicable if the result is not achieved and hardship clauses enabling a party to adapt the contract if circumstances make it too harsh to perform as initially agreed are other examples of contractual terms which may – in one way or another – assist in determining the nature of the obligation in question (see Article 5.1.5(b)).


4. Degree of risk in performance of an obligation

When a party’s performance of an obligation normally involves a high degree of risk it is generally to be expected that that party does not intend to guarantee a result, and that the other party does not expect such a guarantee. The opposite conclusion will be drawn when the desired result can as a rule be achieved without any special difficulty (see Article 5.1.5(c)).



2. A space agency undertakes to put a telecommunication satellite into orbit, the rate of failure of past launchings having been 22%. The space agency cannot be expected to guarantee that the orbiting will be successful. The obligation is merely to observe the degree of diligence required for such launchings in view of the present state of technology.

3. A promises to deliver 20 tons of steel to B on 30 June. Such a relatively simple operation is subject to no special risk. A is committed to the specific result of delivering the required quantity of steel on the date specified and not merely to attempting to do so.


5. Influence of obligee over performance of an obligation

In some situations one party may have a degree of influence over the performance of the other party’s obligations. This fact may transform into duties of best efforts obligations which might otherwise be characterised as duties to achieve specific results.



4. A is prepared to provide B with the technical assistance necessary to apply a newly discovered chemical process, and it is agreed that B will send some of its engineers to attend training sessions organised by A. A cannot promise that the new process will be mastered by the other party, since that result depends in part on B’s effectively sending its engineers to the training sessions, on those engineers’ competence and on their attentiveness at the sessions (see Article 5.1.5(d)).

Where the quality of performance is neither fixed by, nor determinable from, the contract a party is bound to render a performance of a quality that is reasonable and not less than average in the circumstances.



Standards have been set in Article 5.1.4 concerning the exercise of “best efforts”, but quality of performance is a wider problem addressed by Article 5.1.6. If goods are to be supplied, or services rendered, it is not sufficient to supply those goods or to render those services; they must also be of a certain quality.

The contract will often be explicit as regards the quality due (“grade 1 oil”), or it will provide elements making that quality determinable. In other cases, the rule established by Article 5.1.6 is that the quality must be “reasonable and not less than average in the circumstances”. Two criteria are thus combined.



1. A undertakes to build a hotel next to a busy railway station. The contract provides for “adequate sound isolation”, the quality of which is not more precisely determined. It is, however, determinable from the contract that the sound isolation must meet the high standards needed in view of the hotel’s proximity to a railway station.


1. Performance must be of average quality

The minimum requirement is that of providing goods of average quality. The supplier is not bound to provide goods or services of superior quality if that is not required by the contract, but neither may it deliver goods or services of inferior quality. This average quality is determined according to the circumstances, which normally means that which is available on the relevant market at the time of performance (there may for example have been a recent technological advance). Other factors may also be of relevance, such as the specific qualifications for which the performing party was chosen.



2. A buys 500 kgs. of oranges from B. If the contract says nothing more precise, and no other circumstances call for a different solution, those oranges may not be of less than average quality. Average quality will however suffice unless it is unreasonably defective.


2. Performance must be reasonable

The additional reference to reasonableness is intended to prevent a party from claiming that it has performed adequately if it has rendered an “average” performance in a market where the average quality is most unsatisfactory and is intended to give the judge or arbitrator an opportunity to raise those insufficient standards.



3. A company based in country X organises a banquet to celebrate its 50th anniversary. Since the cuisine in country X is mediocre, the company orders the meal from a renowned restaurant in Paris. In these circumstances the quality of the food provided must not be less than the average standards of the Parisian restaurant. It would clearly not be sufficient simply to meet the average standards of country X.

(1) Where a contract does not fix or make provision for determining the price, the parties are considered, in the absence of any indication to the contrary, to have made reference to the price generally charged at the time of the conclusion of the contract for such performance in comparable circumstances in the trade concerned or, if no such price is available, to a reasonable price. 

(2) Where the price is to be determined by one party and that determination is manifestly unreasonable, a reasonable price shall be substituted notwithstanding any contract term to the contrary.

(3) Where the price is to be fixed by a third person, and that person cannot or will not do so, the price shall be a reasonable price.

(4) Where the price is to be fixed by reference to factors which do not exist or have ceased to exist or to be accessible, the nearest equivalent factor shall be treated as a substitute.




1. General rule governing price determination

A contract usually fixes the price to be paid, or makes provision for its determination. If however this is not the case, paragraph (1) of this Article presumes that the parties have made reference to the price generally charged at the time of the conclusion of the contract for such performance in comparable circumstances in the trade concerned. All these qualifications are of course significant. The provision also permits the rebuttal of the presumption if there is any indication to the contrary.

This Article is inspired by Article 55 CISG. The rule has the necessary flexibility to meet the needs of international trade.

It is true that in some cases the price usually charged on the market may not satisfy the reasonableness test which prevails elsewhere in this Article. Recourse would then have to be made to the general provision on good faith and fair dealing (see Article 1.7), or possibly to some of the provisions on mistake, fraud and gross disparity (see Chapter 3, Section 2).

Some international contracts relate to operations which are unique or at least very specific, in respect of which it is not possible to refer to the price charged for similar performance in comparable circumstances. According to paragraph (1) the parties are then deemed to have made reference to a reasonable price and the party in question will fix the price at a reasonable level, subject to the possible review by courts or arbitral tribunals.



1. A, a firm specialised in express mailing throughout the world, receives from B a parcel to be delivered as soon as possible from country X to country Y. Nothing is said as to the price. A should bill B with the price usually charged in the sector for such a service.

2. The next order which A receives from B is one to deliver another parcel as soon as possible to remote and not easily accessible country Z, where a team of explorers is in need of urgent supplies. Again, nothing is said as to price, but since no possible market comparison can be made A must act reasonably when fixing the price.


2. Determination of price by one party

In some cases the contract expressly provides that the price will be determined by one of the parties. This happens frequently in several sectors, for example the supply of services. The price cannot easily be determined in advance, and the performing party is in the best position to place a value on what it has done.

In those cases where the parties have made such a provision for determining the price, it will be enforced. To avoid possible abuses however, paragraph (2) enables judges or arbitrators to replace a manifestly unreasonable price by a reasonable one. This provision is mandatory.


3. Determination of price by third person

A provision that the price will be determined by a third person can give rise to serious difficulty if that third person is unable to accomplish the mission (not being the expert he or she was thought to be) or refuses to do so. Paragraph (3) provides that the price, possibly determined by judges or arbitrators, shall be reasonable. If the third person determines the price in circumstances that may involve fraud, gross disparity or threat, Article 3.2.8(2) may apply.


4. Determination of price by reference to external factors

In some situations the price is to be fixed by reference to external factors, typically a published index, or quotations on a commodity exchange. In cases where the reference factor ceases to exist or to be accessible, paragraph (4) provides that the nearest equivalent factor shall be treated as a substitute.



3. The price of a construction contract is linked to several indexes, including the “official index of charges in the construction sector”, regularly published by the local Government. Several instalments of the price still have to be calculated when that index ceases to be published. The Construction Federation, a private trade association, decides however to start publishing a similar index to replace the former one and in these circumstances the new index will serve as a substitute.

A contract for an indefinite period may be ended by either party by giving notice a reasonable time in advance.



The duration of a contract is often specified by an express provision, or it may be determined from the nature and purpose of the contract (e.g. technical expertise provided in order to assist in performing specialised work). However, there are cases when the duration is neither determined nor determinable. Parties can also stipulate that their contract is concluded for an indefinite period.

This Article provides that in such cases either party may end the contractual relationship by giving notice a reasonable time in advance. What a reasonable time in advance will be will depend on circumstances such as the period of time the parties have been cooperating, the importance of their relative investments in the relationship, the time needed to find new partners, etc.

The rule can be understood as a gap-filling provision in cases where parties have failed to specify the duration of their contract. More generally, it also relates to the widely recognised principle that contracts may not bind the parties eternally and that they may always opt out of such contracts provided they give notice a reasonable time in advance.

This situation is to be distinguished from the case of hardship which is covered by Articles 6.2.1 to 6.2.3. Hardship requires a fundamental change of the equilibrium of the contract, and gives rise, at least in the first instance, to renegotiations. The rule in this Article requires no special condition to be met, except that the duration of the contract be indefinite and that it permits unilateral cancellation.



A agrees to distribute B’s products in country X. The contract is concluded for an indefinite period. Either party may cancel this arrangement unilaterally, provided that it gives the other party notice a reasonable time in advance.

(1) An obligee may release its right by agreement with the obligor. 

(2) An offer to release a right gratuitously shall be deemed accepted if the obligor does not reject the offer without delay after having become aware of it.



An obligee may wish to release the obligor from its obligation (or, in case the obligor owes more than one obligation, from more than one or from all its obligations). The release may either be a separate act, or constitute a part of a more complex transaction between the parties, e.g. a compromise which settles a dispute between them.

This Article provides that such renunciation of the obligee’s right(s) requires an agreement between the parties, irrespective or whether the obligee renounces its right(s) for value or gratuitously.

In the latter case, while the obligor should not be compelled to accept a benefit against its will, it will normally consent to accepting the benefit. For this reason paragraph (2) provides that a gratuitous offer shall be deemed accepted if the obligor does not reject the offer without delay after having become aware of it.



1. Company A is in financial difficulties and needs the co-operation of its creditors in order to survive. Bank B is prepared to renounce 50% of its claim against A and the interest that has fallen due on condition that A pay an interest of 9% (instead of the 5% paid previously) on the remaining debt. B sends a notice to this effect on 15 January. By 22 January A has not reacted to the notice. B’s renunciation will only be effective after A has accepted B’s offer in accordance with Article 2.1.6 et seq.

2. Company A is in financial difficulties and needs the co-operation of its creditors in order to survive. Bank B is prepared to renounce 50% of its claim against A and the interest that has fallen due and sends A a notice to this effect on 15 January. By 22 January A has not reacted to the notice. B’s offer is deemed to be accepted by A.