Scholars of international relations have devoted remarkably little attention to the issue of why and when states enter international organizations (IOs). We argue that states have particular reason to enter IOs during the process of democratization. In the midst of a democratic transition, state leaders have difficulty making a credible commitment to sustain reforms, since they can benefit from rolling back liberalization. Gaining membership in an IO can enhance the credibility of leaders' commitments to democratic reforms. However, not all IOs are equally useful in this regard. We distinguish between IOs that cover standards in areas such as human rights and environmental protection, those that regulate economic activity, and those that are forums for addressing broad political problems. We argue that democratizing states have greater reason to join standards-based and economic IOs than political organizations. The results of a set of statistical tests support this argument.