Three models for the effect of source expertise on attitude change, the heuristic cue model, the evidence model, and the moderator model, were identified and tested. To test predictions based on those models, the effect of source expertise on attitude change and on perceived message-effectiveness were examined with different numbers of supporting arguments and different levels of task importance. The main findings of two studies (total N = 342) are that for low task-importance, as the number of supporting arguments increased, perceived message-effectiveness increased then stabilized, and for high task-importance, as the number of supporting arguments increased, the difference in perceived message-effectiveness between high and low source expertise increased. The evidence model and the moderator model received support.