Knowledge contribution in virtual communities is an important issue in the field of knowledge management. Based on Ames's theoretical framework on motivation, we propose a model for knowledge contribution in problem-solving virtual communities (PSVCs). The model includes two second-order individual motivations, with four major mechanisms in PSVCs that are associated with these motivations. Results confirm that only egoistic motives affect members' knowledge contribution in PSVCs. Further, knowledge repository and social identity are found to be important mechanisms for promoting knowledge contribution through egoistic motives. This paper concludes with theoretical and practical implications and provides insights for future research.