Cohabiters have been shown to invest less in their relationship than married couples. This study investigated the role of legal and interpersonal commitment by examining heterogeneity within marital and cohabiting unions. Going beyond the dichotomy of cohabitation versus marriage, different union types were distinguished by their level of legal and interpersonal commitment, followed by an assessment of their association with joint investments (specialization, having children together, purchasing a home). Using panel data from The Netherlands (N = 2,362), the authors found considerable heterogeneity within marital and cohabiting unions. Joint investments increased as interpersonal commitment increased, with cohabiters without marriage plans investing the least and couples who directly married without prior cohabitation investing the most. The relationship between investments and legal union types was less straightforward and often challenged expectations, prompting the authors to elaborate on alternative explanations for their findings.