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What Is Lex Contractus?
Lex contractus is the law that governs the substance of a contract. It supplies the rules a tribunal applies to decide breach, interpretation, and damages. In international arbitration, the parties usually pick this law themselves in a governing-law clause. That single choice shapes the outcome of any future dispute.
The concept sounds simple. In practice, however, an international arbitration can involve three different laws at once. Confusing them is one of the most expensive drafting mistakes in cross-border contracting. This article explains what the lex contractus does, how it is chosen, and what happens when the contract stays silent.
Key Takeaways
- The lex contractus governs the substance of the contract: validity, interpretation, performance, and damages.
- It is distinct from the lex arbitri (the procedural law of the seat) and from the law governing the arbitration agreement itself.
- Party autonomy rules: tribunals apply the law the parties chose, almost without exception.
- If the contract is silent, the tribunal selects the applicable law — adding cost, delay, and uncertainty.
- Cases like Enka v Chubb and Kabab-Ji v KFG show how badly things go when the governing-law clause leaves gaps.
One Arbitration, Three Laws
Keep three laws apart:
| Law | What it governs | Where it comes from |
|---|---|---|
| Lex contractus | The substance: interpretation, breach, remedies | The governing-law clause |
| Lex arbitri | The procedure: court supervision, set-aside, interim relief | The seat of the arbitration |
| Law of the arbitration agreement | Validity and scope of the agreement to arbitrate | Ideally stated expressly; otherwise inferred |
The three can differ in a single case: a contract governed by New York law, seated in London, with the arbitration clause governed by English law. Each law answers different questions, and tribunals switch between them constantly. Most drafting disasters come from assuming that one governing-law clause automatically covers all three.
How Parties Choose the Lex Contractus
Party autonomy is the first principle: arbitration laws and institutional rules direct tribunals to apply whatever law the parties chose. The choice is strategic. Parties weigh the legal system’s predictability and its case law on their industry’s contracts. They also weigh its approach to damages and penalties, and its neutrality between the two sides. English and New York law dominate international commerce for exactly these reasons.
The parties need not choose a national law at all. They may select non-national standards such as the UNIDROIT Principles of International Commercial Contracts, alone or alongside a national law. In addition, sales contracts should say expressly whether the CISG applies. It applies by default in most trading nations, so silence means opting in without noticing.
What Happens When the Contract Is Silent
If the parties never chose a law, the tribunal chooses for them. Under Article 28 of the UNCITRAL Model Law, the tribunal applies the law determined by the conflict-of-laws rules it considers applicable. The ICC Rules go further and let the tribunal apply directly the rules of law it determines appropriate — the so-called voie directe.
Either way, the parties have handed a critical decision to strangers. The tribunal may look to the place of performance, the parties’ locations, or the contract’s center of gravity. The parties then spend a procedural round briefing the question. Consequently, the cheapest insurance in international contracting is a one-sentence governing-law clause.
How the Lex Contractus Shapes the Arbitration
Once the case begins, the lex contractus does the substantive work. It supplies the rules of contract interpretation, decides whether a breach occurred, fixes the measure of damages, and determines defenses such as force majeure or hardship. Two tribunals hearing identical facts under different governing laws can reach different results — on penalty clauses, on consequential damages, on good-faith obligations. That is why the choice matters commercially, not just legally.
Procedure is a different story. Evidence, hearings, court assistance, and challenges to the award follow the lex arbitri of the seat, however the contract’s substance is governed. The two systems run in parallel throughout the case.
When Laws Collide: Enka v Chubb and Kabab-Ji
Two recent English cases show what happens when the contract does not say which law governs the arbitration agreement itself. In Enka v Chubb (2020), the UK Supreme Court set the default rule. Absent an express choice, the law chosen for the contract generally extends to the arbitration agreement. However, the seat’s law takes over where that choice would invalidate the clause.
Kabab-Ji v KFG made the stakes concrete. The contract was governed by English law with a Paris seat. The English courts applied English law to the arbitration agreement and refused to enforce the award against the non-signatory parent company. The French courts, applying French law as the law of the seat, upheld the same award. One arbitration, two opposite outcomes — caused entirely by a drafting gap. Enforcement risk of this kind is exactly what our guide to enforcing foreign arbitral awards warns about.
Drafting Best Practices
A clean set of clauses avoids nearly all of the problems above:
- State the lex contractus expressly: “This contract is governed by the laws of [jurisdiction].” Name a specific legal system, not “U.S. law.”
- State the law governing the arbitration agreement separately — one sentence prevents an Enka or Kabab-Ji dispute.
- Fix the seat deliberately, because it brings the lex arbitri and the supervising courts with it.
- Say whether the CISG applies, and whether trade usages or the UNIDROIT Principles supplement the chosen law.
These lines belong in the first draft, not the final one — they are rarely contested when raised early. Our guide to drafting international arbitration clauses covers the full checklist. Our comparison of international arbitration vs litigation helps decide whether arbitration is the right forum at all.
How Transnational Matters Can Help
Our firm advises on governing-law and arbitration clauses at the drafting stage. We also argue lex contractus questions in arbitration, from choice-of-law disputes to enforcement fights across jurisdictions. If your contract’s governing-law architecture needs review, or a dispute has already exposed a gap, contact our office.
Conclusion
The lex contractus decides what a contract means and what its breach costs, which makes it the most consequential single choice in an international agreement. Choose it expressly, and pair it with a stated seat and a stated law for the arbitration agreement. The arbitration will then run on rails the parties laid themselves. Leave it silent, and a tribunal will choose for you — after briefing, delay, and expense that one sentence would have avoided.
The law governing your contract can shape every stage of an international arbitration, from interpretation to enforcement. Contact our team to review the governing-law provisions in your international agreements.
