This paper aims to better understand the dynamic aspects of strategic alliances between competitors, by relying on both the real options approach and the system dynamics approach. It raises the following question: what are the behavioral dynamics of real options in alliances? The systemic qualitative model we built from the study of an alliance formed by two competing firms around an R&D project revealed three sets of feedback structures (the "expected flexibility", "uncertainty" and "collaborative skills and knowledge" dynamics), in which are embedded three types of real growth options (that is, unexpected discovery, relational reputation and interactive options). While previous studies have often focused on how partners unilaterally manage real options to reduce uncertainty, our study allowed us to identify the real options created by the alliance, which can be exploited collectively, independently by each partner or not at all, and to capture their dynamic aspects.