This article measures the effects of various levels of interactivity and vividness of a message on attitudes and behavioral intentions within a web-based advertisement. As a conceptual foundation, the study introduces the multistep model of the impact of interactivity on advertising effectiveness. The model is termed multistep because of the hierarchy of direct and indirect effects. A 3x3 interactivity by vividness between-subjects factorial design tests the model. A total of 360 responses were collected through an online web interface. Data were analyzed by means of individual analysis of covariance (ANCOVA) procedures. The multistep model was also tested with path analysis to verify the significance of the interrelationships between constructs in a simultaneous equations procedure. Results indicate moderate effects of interactivity and vividness on social presence and, indirectly, involvement that in turn have strong effects on traditional advertising effectiveness measures. The findings suggest that the effects of interactivity reach a ''plateau'' at medium and high levels, indicating a diminishing returns effect. Conversely, the impact of vividness appears to be linear with a steady increase across low, medium, and high levels. No interaction effect was found between the two treatments. The study also provides some insights on using the web as a gateway for experimental research and data collection.