We used data from two waves of the National Survey of Families and Household to investigate changes in mothering behavior associated with remarriage or cohabitation by single mothers. We considered three dimensions of mothering: (a) time and supervision, (b) harsh discipline, and (c) relationship quality. Mothers and children agreed that mothers who remained in new partnerships used harsh discipline less frequently than mothers who remained single or whose new partnership had ended by the second interview. Mothers reported less supervision if they had experienced a disrupted partnership, whereas children reported less supervision if their mothers remained in a new partnership at the second interview. Children but not mothers reported better relationships with mothers in partnerships at the second interview, compared with children whose mothers remained single or whose new partnership had ended. Only a small part of the differences in harsh discipline, and none of the other observed differences, could be explained by maternal or family characteristics or by mothering behavior and relationships in the first interview. Although cohabiting partnerships were more likely to end than were marriages, we found no differences in effects of cohabiting or marital partnerships, net of their status at the second interview.