Illinois Postnuptial Agreement

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Updated August 07, 2022

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An Illinois postnuptial agreement is a legal document that lets couples determine the distribution of marital property if their marriage ends in divorce or death. Unlike a prenuptial agreement (commonly known as a “prenup”), which is signed before a marriage begins, a postnuptial agreement is signed after a couple marries. Both documents can help couples shield themselves against the uncertainty that can result when a divorce judge has wide latitude to distribute the couple’s assets.

Signing Requirements (In re Marriage of Richardson (1992)) – Both parties must sign the agreement.


Marital Property: Property excluded by a valid agreement of the parties, including a postnuptial agreement, is considered “non-marital property.” 750 ILCS 5/503(a)(4).

Adequate Consideration: Marriage itself is sufficient consideration for a valid postnuptial agreement. In re Estate of Brosseau (1988):

Duress: Duress may be enough to render an agreement between spouses unconscionable. Duress includes

  • Oppression
  • Undue influence, or
  • Taking undue advantage to the point where another cannot exercise free will.

Evidence of duress must be clear and convincing before a court may set aside a postnuptial agreement. In re Marriage of Richardson (1992).