The aim of this scientific paper is to determine the extent to which social identity and organizational trust mediate th e relationship between the informal institutions and the knowledge sharing. Based on a review of literature and relying on the theoretical perspective of sociological neo - institutionalism and on the resource based - view, the research hypothesis was formulat ed. Using a cross - sectional and with Mexican public sector organizations and through the perceptions of 252 public servants the hypothesis was tested. Multivariate analysis (SEM) was performed. The results confirm that the informal institutions legitimize the performance of organizations but do not guarantee its real efficiency. However, research shows that intervening valuable resources management (social identity and organizational trust) helps to take advantage of opportunities and neutralize threats fro m the environment generating new capabilities: the knowledge sharing.