This longitudinal study followed 120 cohabiting, opposite-sex couples over 8 months to test hypotheses derived from commitment theory about how two types of commitment (dedication and constraint) operate during cohabitation. In nearly half the couples, there were large differences between partners in terms of dedication. These differences were associated with lower relationship adjustment, even controlling for overall level of dedication. Furthermore, among couples who believed in the institution of marriage, cohabiting women were, on average, more dedicated than their partners. Additionally, there was evidence that constraints (e.g., signing a lease, having a joint bank account) make it less likely that couples think they will break up, regardless of relationship dedication. This finding was strongest for women and for those with higher income levels. (Contains 3 tables and 2 notes.)