Although numerous studies have examined the relationship between college diversity experiences and student outcomes, very little attention has been paid to the mechanism(s) underlying this development. This paper proposes a model of the psychological process through which college diversity experiences affect student attitudes and examines that model empirically using a sample of 387 students who completed service-learning courses. Structural equation modeling analyses indicate that diversity interactions are associated with "experiencing the unexpected," which then contributes both to a subjective sense of belief challenge and to attitude change. Implications for practice and future research are discussed. (Contains 2 figures, 2 tables, and 2 footnotes.)