Motivating one’s workforce is a major challenge for organizations. Demotivated employees not only cause harm to organizations’ productivity and innovation but also show increased turn-over intentions. As in the case of organizations, projects developing Free Libre Open Source Software (FLOSS) depend strongly on the motivation of their contributors. Existing research repeatedly highlights project members’ motivation for FLOSS projects’ success and continuance. However, existing evaluations primarily treat project members’ participation motives as exogenous constructs. As a result no operational advice can be derived for FLOSS projects on how to motivate their contributors. This research takes an alternative view and regards FLOSS developers’ motivation as product of their self-determination and influences of their surrounding environment. Drawing on Self-Determination-Theory (SDT), we consider FLOSS developers’ motivation as multi-dimensional and sensitive to environmental stimuli. As an example for such environmental stimuli we propose that project celebrities (members with a high standing in the FLOSS community) stimulate the participation motives of project members in different ways. An evaluation with 65 participants of Google Summer of Code (GSoC) largely supports our research model and provides evidence for the endogenous character of FLOSS developers’ motivation. Our research results suggest that celebrities stimulate rather self-determined than externally regulated motives.