In recent years, voluntary associations and political organizations have increasingly switched to Internet-based mobilization campaigns, replacing traditional forms of face-to-face recruitment and mobilization. The existing body of empirical research on Internet-based mobilization, however, is not conclusive about the effects this form of mobilization might have. In this article, the authors argue that this lack of strong conclusions might be due to the failure to distinguish different behavioral outcomes of mobilization, and more specifically, a distinction between online and off-line forms of participation is missing. In this experimental study, participants were exposed to potentially mobilizing information either by way of face-to-face interaction or by website. The results of the experiment indicate that web-based mobilization only has a significant effect on online participation, whereas face-to-face mobilization has a significant impact on off-line behavior, which would imply that mobilization effects are medium-specific. The authors close with some observations on what these findings might imply for the democratic consequences of the current trend toward an increasing reliance on Internet-based forms of political mobilization.