Data from a two-wave survey of low-income families in Boston, Chicago, and San Antonio are used to replicate recent reports of a modest increase in the number of low-income children living in two-adult families and to analyze the increase. We find that most of the increase occurred through the addition of a man other than the biological father to the household and that more of it occurred through cohabitation than through marriage. Moreover, across the two waves, cohabiting and marital unions were highly unstable. We review research on stepfamilies and on instability in children’s living arrangements, and we conclude that the kinds of two-adult families being formed in these low-income central-city neighborhoods may not benefit children as much as policy-makers hope. In addition, we investigate the associations between marital and cohabiting transitions, on the one hand, and transitions into and out of Temporary Assistance to Needy Families (TANF) receipt, employment, and Earned Income Tax Credit (EITC) usage between the two waves on the other. We find that marital transitions are related to TANF and employment transitions but that cohabiting transitions are not. We suggest that low-income mothers may view marriage as more of an economic partnership than cohabitation and may expect more of an economic contribution from a husband than from a cohabiting partner.