This study focuses on the factors underlying differences in relationship quality between interethnic and same-ethnic couples. Using the National Survey of Families and Households and the Fragile Families and Child Wellbeing Study, we examine relationship satisfaction, interpartner conflict and subjective assessments of relationship instability in married and cohabiting couples. Partners in interethnic unions generally reported lower levels of relationship quality than did partners in same-ethnic unions. These differences held for women as well as men, and for married as well as cohabiting couples. Differences in relationship quality were largely accounted for by more complex relationship histories, more heterogamous unions, fewer shared values and less support from parents. In contrast, differences in socioeconomic resources did not appear to play an explanatory role. (Contains 6 tables.)