Effects of intervention with the Oregon model of Parent Management Training (PMTO) on marital relationship processes and marital satisfaction in recently married biological mother and stepfather couples were examined. Sixty-seven of the 110 participating families were randomly assigned to PMTO, and 43 families to a non-intervention condition. Intervention had reliable positive indirect effects on marital relationship processes 24 months after baseline which in turn were associated with higher marital satisfaction. These indirect effects were mediated by the impact of PMTO on parenting practices 6 months after baseline. Enhanced parenting practices resulting from PMTO prevented escalation of subsequent child behavior problems at school. Consistent with a family systems perspective and research on challenges to marital quality in stepfamilies, improved co-parenting practices were associated with enhanced marital relationship skills and marital satisfaction as well as with prevention of child behavior problems.