The paper concentrates on the role personal norms play in the decision to use public transportation means instead of the car. An integrative theoretical framework is presented from which hypotheses concerning the social and psychological processes activating personal norms and mediating their impact on behavior are derived. These hypotheses are tested and cross-validated with two independent data sets collected in two German urban agglomerations with different economic and social-cultural backgrounds. In both samples, personal norm is a significant predictor of PT-use intention even after checking for the impact of attitude and perceived behavioral control. Furthermore, there is evidence that the reported personal norms are based on two related but distinct processes: anticipated feelings of guilt and perceived social norms. These two processes provide an explanation how different social contexts may influence people's judgment of PT as well as their actual PT-use.