The assimilation model of therapeutic change describes the self as comprised of multiple internal voices (mental states), and tracks the development of understanding and joint action between those voices in successful psychotherapies. This model has been constructed from studies of individual therapy, and has generally focused on intrapersonal change. The emotion-focused, couple therapy case presented here involving Sarah and Mark was studied using an iterative, team-based approach to theory-building case-study research to (1) assess whether the process of resolving interpersonal problems in couple therapy is isomorphic to the process of assimilation in individual therapy and, if possible, and to (2) extend the model's ability to describe the process of resolving interpersonal problems. Observations supported the hypothesis that intra-and interpersonal problems resolve in ways that are isomorphic to one another: The patterns of perception, emotion, communication, and action observed during the process of resolving interpersonal problems were consistent with those observed in intrapersonal problem resolution in previous assimilation model research. As the couple developed mutual understandings and increasingly engaged in cooperative action, previously excluded aspects of one another's experience came to be increasingly included (i.e., valued and influential) in each partners' meaning-and decision-making. This parallels the increasing inclusion of previously avoided experience observed in successful individual treatments. Case observations are presented, and clinical and research implications are discussed.