This paper studies the role of social influence in the adoption of information technology. Previous studies have demonstrated the crucial role of social interactions in the case of Open Source Software adoption behaviors for individuals. However, the literature reveals ambiguity on the definition and the measurement of social influence concept. Thus, this paper attempts to develop a kind of understanding about this ambiguity by clarifying the sources of social influence in organization: voluntary use, image, visibility, normative influence, conformity motivation. These concepts identified already in the literature are summarized into two second order constructs in the research model: group pressure and social influence. The research model developed aim to explain and predict Open Source Software adoption behavior depending on these two concepts. Data validation using the Partial least square approach has uncovered new and thorough theoretical and empirical results in the field of information technology/system adoption.