Using data from the 1987–1988 National Survey of Families and Households, this paper studies the role of the religious composition of unions as a determinant of marital stability. With the exceptions of Mormons and individuals with no religious identification, stability is found to be remarkably similar across the various types of homogamous unions. Consistent with the notion that religion is a complementary marital trait, interfaith unions have generally higher rates of dissolution than intrafaith unions. The destabilizing effect of out-marriage varies inversely with the similarity in beliefs and practices of the two religions as well as with the mutual tolerance embodied in their respective doctrines. The results also suggest that religious compatibility between spouses at the time of marriage has a large influence on marital stability, rivaling in magnitude that of age at marriage and, at least for Protestants and Catholics, dominating any adverse effects of differences in religious background.