For many nonprofit organizations, it is of the utmost importance to understand why some volunteers donate more time than others. This article ex-amines how the Big Five personality traits relate to the amount of time donated by volunteers and proposes that transactional, relational, and ideological psycho-logical contracts mediate this relationship. Moreover, this study examines whether the interaction among extraversion, agreeableness, and tenure explains additional variance in the ideological contract. Path analysis is used to estimate a moderated mediation model, on the basis of data from two time-lagged surveys (N 5 458). The results reveal direct relationships between personality traits and the three psy-chological contract types and support hypothesized interactions in explaining the ideological contract. This study also finds that transactional and relational contracts act as mediators. The authors conclude that psychological contract types can help explain why, on the basis of personality differences, some volunteers donate more time to their nonprofit organization than others.