This article introduces the actor–partner-interdependence–investment model (API-IM) that was developed to add a dyadic perspective to Rusbult's investment model. The API-IM is based on interdependence theoretical assumptions and the actor–partner interdependence model. Two studies were conducted to investigate the reliability of the API-IM. Relationship satisfaction, investment size, quality of alternatives, and relationship commitment were assessed at both partners of 77 (Study 1) and 162 (Study 2) married and unmarried heterosexual couples. Path analyses that applied a structural equation modeling framework revealed a dyadic model that significantly predicts women's and men's commitment by actor effects of satisfaction, investments, and alternatives, and partner effects of satisfaction. Actor and partner effects of satisfaction were significantly moderated by relationship duration and marital status. Marital status also significantly moderated the actor effect of alternatives. The API-IM supports the concept of social interdependence in close relationships, and it is discussed as a sound dyadic extension of the investment model. Copyright © 2012 John Wiley & Sons, Ltd.