In many states twenty-five years ago, prenuptial agreements (i.e., those made prior to a marriage) were not binding. Since then, most states have accepted the legality of prenuptial agreements, providing certain requirements are satisfied. The rules vary from state to state, but generally, a premarital agreement must entail a knowing waiver of marital rights, must be fair and reasonable (or at least not unconscionable), and not be coercive.
Law is being developed now about the validity and enforceability of postnuptial agreements. There are variations from state to state in the requirements for a binding postnuptial agreement. Factors indicating enforceability would be a postnuptial agreement that is non-coercive, parties each had a separate reviewing attorney, and if a party is not secretly motivated to better the terms of a divorce. Also, equitable principles are important, and the terms of the postnuptial agreement should be fair and reasonable, almost approaching the type of terms in a divorce agreement that a court would approve.
Because the laws vary from state-to-state, it is important for you (or your attorney) to review the law in your state before embarking on the process of formulating a postnuptial agreement. Also, postnups can be dangerous — if over-reaching, they have the effect of destroying a marriage rather than promoting and sustaining it.
But even if the ultimate enforceability is not settled in your jurisdiction (or under the particular facts of your marriage and the agreement), a postnuptial agreement, if fair, can be upheld by the parties, and can give them relief if the ways they want, so that their marriage can continue and be ongoing.