Abstract: Web site interactivity creates numerous opportunities for marketers to persuade online consumers and receives extensive attention in the marketing literature. However, research on cognitive and behavioral responses to web site interactivity is scarce, and more importantly, it does not provide empirical evidence for how interactivity effects can be explained. Therefore, the aim of this study was to investigate the underlying principle that explains the influence of web site interactivity on consumers’ cognitive, affective and behavioral responses: online flow, the web site users’ complete immersion in an online activity (Hoffman and Novak 2009). In two studies, the hypothesis was tested that a visitor’s flow experience in a specific brand web site mediates the effects of interactivity on the number and type (web site vs. product related) of thoughts, on attitudes toward the brand and web site, and on several behavioral intentions. The results provide evidence for the importance of flow in a marketing context, and the notion that the flow experienced on a specific web site is the underlying mechanism by which cognitive, attitudinal, and behavioral responses to an interactive brand web site can be explained.