The article provides the results of research carried out within the organizational social capital theory and the network theory of organizations. The research contributes to these theories by identifying, through an analysis of the literature, 10 informal network sources of intra-organizational social capital. A model for representing and measuring intra-organizational social capital based on the 10 informal organizational networks identified and on three different conceptual perspectives (structural, relational and cognitive) is subsequently proposed. The model is validated through an empirical study conducted within five divisions of a knowledge-intensive company. The research results suggest that among the 10 informal relationships studied, the accessibility of individuals primarily predicts the emergence of more informal relationships, which are the real source of intra-organizational social capital through resource exchange and combination. Furthermore a continuous working cooperation and the willingness of colleagues to make themselves available for cooperation also create a shared organizational culture in terms of language, norms and values and maintain a stable working information flow within organizations.