The present study investigated roles of common factors related to outcome indices that exist across therapy models in couple therapy for partner aggression. As client common factors, individuals' pre-therapy levels of trust in their partners and degrees to which they vent anger were tested as predictors of change in relationship satisfaction and psychological aggression. As therapist factors degree of empathy expressed toward clients, use of systemic intervention techniques, and degree to which the therapist imposed structure on sessions were examined in relation to therapy outcomes. Based on social learning theory, not only main effects but also interaction effects of client factors and therapist factors on therapy outcomes were examined. Structural equation modeling was used to test an Actor-Partner Interdependence Model in which partners scores on measures of relationship qualities are assumed to influence each other. Female partners benefited more from the therapy than did male partners, particularly in increases in relationship satisfaction. However, females and males had equal reductions in psychological aggression. Unexpectedly, lower level of trust predicted more positive change in psychological aggression, but not in relationship satisfaction. As expected, higher venting of anger was negatively associated with improvement in relationship satisfaction and psychological aggression. Higher therapist use of systemic techniques predicted more positive change in relationship satisfaction only for female partners. Interaction effects suggesting a buffering role of empathy against the negative effect of a lower level of trust were detected. Overall, it was client factors and not therapist factors that made differences in therapy outcome indices. Also, predominantly actor effects rather than partner effects occurred. That is, clients' changes on outcome indices were related more to their own characteristics than to their partners' characteristics. Strikingly, regarding partner effects, only male-to-female partner effects were found, meaning that female partners' therapeutic changes over the course of therapy were predicted by their male partners' characteristics. Overall, the present study demonstrated that common factors do operate in couple therapy for partner aggression across therapy models, although those common factors are mostly client characteristics rather than therapist behaviors and involve male partners' characteristics.