Unilateral agreements

Most contracts are bilateral in the sense that X and Y each promise to give or do something. But with unilateral contracts it is only the offeror that makes a promise. For example, if X says, ‘I will pay a £100 reward to anyone who finds and returns my gold ring to me’, he has made a promise that is not reciprocated by anyone, since nobody is bound to find and return his gold ring. Such contracts are sometimes referred to as ‘if’ contracts as in: ‘If you do X, I promise to do Y’. Moreover, whereas a bilateral agreement is a promise in return for a promise, a unilateral agreement is a promise in return for an act (unsupported by any promise to actually do the act).

Due to their different nature unilateral contracts have different rules regarding:

Unilateral advertisements are likely to be construed as offers rather than invitations to treat.

Acceptance must be in response to the offer, and with knowledge of it. There is no binding contract if a person just happens to find the missing ring and, since he knows the owner, returns it to him.

Revocation. A unilateral offer may not be revoked once the promisee has partially performed, or started to perform, the required act. For example, if X makes a unilateral offer to pay £100 to anyone who walks from London to Newcastle, X cannot revoke the offer whenever anyone gets close to Newcastle. (1)

(1) Carlill v Carbolic Smoke Ball Company [1893] 1 QB 256, CA.