Who Has the Burden of Proof to Set Aside a Domestic Contract in Ontario?

You signed a marriage contract, cohabitation agreement, or separation agreement at some point, and now one of you wants a court to say it shouldn’t count. Before that conversation goes any further, it helps to know how Ontario law actually treats a challenge like this: who has to prove what, and how high that bar really is.

Domestic contracts are treated as ordinary, binding contracts once they meet Ontario’s basic signing requirements. That means the person who wants a court to set one aside — not the person defending it — carries the burden of proof to set aside a domestic contract. Simply disliking how the agreement turned out is not enough on its own.

This guide walks through who has to prove what, the grounds Ontario courts actually consider, and the kind of evidence that tends to matter.

Contracts Are Presumed Valid Once Properly Signed

Under Ontario’s Family Law Act, a domestic contract — a marriage contract, cohabitation agreement, or separation agreement — is valid as long as it is in writing, signed by both parties, and witnessed. No court approval is required to sign one.

Once those formalities are met, Ontario courts start from the same place they would with any other contract: it is presumed valid and enforceable. The agreement does not need to be reviewed or endorsed by a judge to take effect, and it will not be reopened automatically just because circumstances changed or one party later regrets a term.

Who Has to Prove What

The party who wants the contract set aside is the one who has to prove it. In practice, that means:

• If you signed the agreement and now want out of it, you carry the burden of establishing a recognized legal ground for setting it aside.
• If you are relying on the agreement — for example, defending its terms in a support dispute — you generally do not need to prove the agreement was fair. You can rely on the presumption of validity unless the other side proves otherwise.

This matters because it shapes strategy from the start: a challenge built on "I don’t like this anymore" will not get far, while one built on a recognized legal defect has a real chance.

Common Grounds Used to Challenge a Domestic Contract

Ontario courts will consider setting aside a domestic contract, in whole or in part, where the challenging party proves one of a small number of recognized problems, including:

• Lack of financial disclosure — one party did not disclose significant income, assets, or debts at the time the contract was signed, and the other party did not otherwise know about them.
• Lack of understanding — a party did not understand the nature or consequences of what they were signing.
• General contract law defects — the same problems that can undo any contract, such as duress, undue influence, unconscionability, or misrepresentation.

Each of these requires evidence, not just an assertion. A party alleging non-disclosure, for example, typically needs to show what was hidden and that it was significant enough to matter.

What Courts Do Not Treat as Grounds on Their Own

It helps to know what usually will not succeed:

• The agreement turned out to be a worse deal than expected once circumstances changed.
• One party now earns more, or less, than they did when they signed.
• The agreement feels unequal or one-sided — without more, a lopsided outcome is not automatically an invalid one.

Ontario law generally holds people to the bargain they made, even an imperfect one, unless a recognized legal defect can be proven.

Evidence That Tends to Matter

If you’re considering a challenge, or defending against one, the following typically shapes how a court views the case:

• [ ] Financial statements or disclosure exchanged before signing
• [ ] Whether each party had independent legal advice, and from whom
• [ ] How much time passed between receiving the draft and signing it
• [ ] Any documented pressure, deadlines, or circumstances around the signing
• [ ] Correspondence showing what each party understood the contract to mean

None of these factors is decisive on its own, but together they help a court decide whether the presumption of validity should hold.

Frequently asked questions