The article reflects on the validity of canonical marriage with respect to separation of property determined by prospective spouses under Polish law by way of the so-called prenuptial agreement. The analysis of the norms of Polish law regulating the separation of property and seeking corresponding norms in canon law are intended to answer whether a prenuptial agreement made before marriage can constitute an exclusion of one of the aspects of the community of life and thus may have a probative value in a nullity case. The present analysis affords the conclusion that it is not the prenuptial agreement, but rather the intention with which the prospective spouses concluded it that determines the validity of canonical marriage. Indeed, signing such an agreement may sometimes be objectively justified, legitimate or even advisable. However, if it demonstrates a party’s unwillingness to build marital unity, it will be a reason for declaring the marriage invalid on at least three counts: 1) as exclusion of conjugal indissolubility (bonum sacramentum), 2) as incapacity to assume the essential obligations of marriage, and 3) as exclusion of the good of the spouses (bonum coniugum).