Research has rarely considered intermediate variables in the association between attachment insecurities and relationship commitment. Based on attachment theory and Rusbult's Investment Model, we tested whether positive and negative partner behaviors explain this association in 91 distressed couples undergoing therapy. Path analyses based on the Actor-Partner Interdependence Model revealed that individuals with attachment avoidance reported perceiving fewer positive behaviors from their partner, which was associated with their own higher under-commitment. Partners of individuals with avoidance also reported perceiving fewer positive partner behaviors, which was in turn associated with their own higher under-commitment. Partners of individuals with attachment anxiety perceived more positive behaviors from their partner, which was associated to their own lower under-commitment. Perceived negative partner behaviors were only directly associated with higher under-commitment. These findings imply that perceived positive partner behaviors may be a key aspect to target in relationship therapy in couples who present with high attachment insecurities.