We suggest that thematic ad–medium congruency versus incongruency evokes distinct effects on consumer evaluations through different underlying mechanisms. Specifically, we propose congruency largely has positive effects on consumer evaluations due to a relatively automatic fluency process, whereas incongruency evokes a more conscious and negative persuasion knowledge (PK) process that leads to negative evaluations. Study 1 showed that consumers were more attentive to incongruence than congruence, particularly when the ad–medium combination was presented with other ads or materials. Studies 2A and 2B confirmed that congruency led to positive evaluations through perceived fluency, whereas incongruency led to negative-PK thoughts involving manipulative intent and more negative evaluations. Studies 3A and 3B provided causal evidence for the role of PK by showing that positive PK attenuated the negative effects the incongruency tactic had otherwise. Overall, these findings suggest the common practice of directly comparing congruent and incongruent media tactics confounds two very different processes. Managerial implications for advertising and media marketing are discussed.