Unit nonresponse is a critical issue in survey methodology. The purpose of this paper is to understand better the mechanisms that underlie unit nonresponse; i.e., the individual's decision to participate in online panel surveys. Based on the theory of planned behavior (TPB), the determinants of unit nonresponse are one's attitude toward participating in online panel surveys, the subjective norm, and perceived behavioral control. Using a student sample at the University of Berne (N = 308), our results show only a marginal effect of the intention to predict participation behavior, and no effect was found for perceived behavioral control. Instead, additional explanatory variables (e.g., moral obligation, frequency of past behavior) led to a better explanatory power of the TPB.