This study contributes to the emerging demographic literature on same-sex couples by comparing the level and correlates of union stability among 4 types of couples: (a) male same-sex cohabitation, (b) female same-sex cohabitation, (c) different-sex cohabitation, and (d) different-sex marriage. The author analyzed data from 2 British birth cohort studies: the National Child Development Study (N = 11,469) and the 1970 British Cohort Study (N = 11,924). These data contain retrospective histories of same-sex and different-sex unions throughout young adulthood (age 16-34) from 1974 through 2004. Event-history analyses showed that same-sex cohabitations have higher rates of dissolution than do different-sex cohabiting and marital unions. Among same-sex couples, male couples had slightly higher dissolution rates than did female couples. In addition, same-sex couples from the 1958 and 1970 birth cohorts had similar levels of union stability. The demographic correlates of union stability are generally similar for same-sex and different-sex unions. (Contains 4 tables and 1 figure.)