A post-nuptial agreement is basically the same as its more well known counterpart, the pre-nuptual agreement. The only essential difference is in the timing (after instead of before the marriage.)
In many cases, post-nuptual agreements have the same purposes as pre-nuptuals; however, this is not always the case. While it may be that you didn't have time to get a pre-nuptual agreement before getting married or hadn't thought of it, but now you simply want to address financial and inheritance issues; sometimes, it's a matter of seeking protection because the other spouse seems out of control with spending or in some other way.
But just like pre-nuptual agreements, post-nuptuals must be consented to by both parties and be put into writing. Oral agreements can't count here legally. And the agreement cannot be overly one-sided, done for threat or fraud, or done without both spouses giving full disclosure of all income, debts, and assets.
