This study refines and experimentally tests a theory of relational cohesion that explains how and when actors become committed to one another in the context of multiactor exchange. The theory asserts that frequent social exchange results in (1) positive emotions that solidify and strengthen the person-to-group bond and (2) uncertainty reduction that renders the focal group more salient in relation to others. These two mechanisms produce a sense of psychological group formation and ultimately increase observable acts of commitment. In a “productive exchange” setting, three actors negotiate a joint venture that requires the assent of all members. The exchanges featured two forms of commitment behavior: the giving of small token gifts and the decision to invest in a three-way prisoner’s dilemma. The results suggest that positive emotion and uncertainty reduction are theoretically distinct and affect commitment behavior through different mechanisms. The article concludes by discussing the general implications for commitment and social order.