Increasing knowledge contribution in problem solving virtual communities (PSVCs) is a long-pursued question in the knowledge management arena for both researchers and practitioners. In view of various mechanisms developed in hopes of encouraging more active participations, we proposed a motivational model for PSVC contributions. Drawn on functional motivation theory and expectancy-value theory, we highlight the mediating role of individual motivations in the relationships between characteristics of PSVCs and knowledge contribution intention. By taking both self-interest and public-good perspectives, we identify eight individual motivations to knowledge contribution in the context of PSVCs. They are active learning, self-enhancement, reciprocity, reputation, enjoyment of helping others, self-protection, moral obligation and advancement of virtual community motive. Besides, we also identify four major mechanisms in PSVCs that are associated with members' motivations: codification, identity management, virtual community norms and communication methods.