This paper argues for a clearer conceptualization of media stimuli in experimental research and identifies 3 issues impeding our understanding of message processing: (a) assumptions bolstered by manipulation checks about homogeneity of response to media stimuli, (b) conflation of 2 different classes of variables--media attributes and psychological states, and (c) discrepancies between the conceptual model and operational-level hypotheses used to test research questions. To provide a more comprehensive framework for investigating media effects in experimental research, we argue for a clearer conceptual separation between message attributes and user perceptions and apply a mediation model of information processing to overcome the limitations of conventional approaches. Subjected to 2 empirical tests involving the assessment of Web-based media, the model finds an increase in explained variance in each instance.