Interdependence theory identifies level of dependence and mutuality of dependence as two key properties of interdependent relationships. In ongoing relationships, these structural properties are subjectively experienced in terms of commitment–dependence level is experienced as greater or lesser commitment level, and mutuality of dependence is experienced as greater or lesser perceived mutuality in partners’commitment levels. We examined the associations of these variables with couple well-being using data from two three-wave longitudinal studies. One study examined partners in dating relationships and the second study examined partners in marital relationships. Consistent with predictions, both level of commitment and perceived mutuality of commitment accounted for unique variance in couple well-being: Couples exhibited greater adjustment to the degree that the partners were highly committed to their relationship and to the degree that their commitment levels were mutual. Mediation analyses revealed that the association of mutuality of commitment with couple well-being is partially mediated by negative affect (e.g., anxiety, guilt) and partially to wholly mediated by trust level; perceived mutuality of power is not a reliable mediator of this association.