A major challenge for the creators of international environmental agreements is how to design mechanisms that deter defection without deterring participation. Indeed, relatively 'soft' international law often garners widespread participation, but it creates few concrete incentives for states to improve behavior. 'Harder' legal commitments make shirking more difficult, but these institutional features may deter from joining the very states whose environmental practices are least consistent with the treaty's requirements. I argue that these institutional features have important consequences for the prospects of ratification: the harder a treaty is, the more 'selective' states are about ratification. Empirical analyses of ratification of the UN Framework Convention on Climate Change and the Kyoto Protocol provide substantial support for this proposition. As others have noted, one way in which states can mitigate this 'dilemma' that legalization produces is by creating agreements with flexibility provisions. I argue that states' ability to use these provisions increases their propensity to ratify, even if it does not fundamentally alter the relationship between compliant behavior, legalization, and ratification. The empirical findings with regard to one flexibility mechanism - carbon sinks - support this argument. The results with regard to Activities Implemented Jointly, however, follow an opposite pattern, suggesting that states may sometimes use flexibility provisions as a means of legitimizing (or at least attracting attention away from) their non-participation in the regime. Finally, I explore how social networks affect ratification. I find consistent evidence that domestic and international networks matter for ratification of the Framework Convention, but not the Kyoto Protocol. ..PAT.-Unpublished Manuscript [ABSTRACT FROM AUTHOR]