Civil society is commonly assumed to have a positive effect on international cooperation. This paper sheds light on one important facet of this assumption: we examine the impact of environmental non-governmental organizations (ENGOs) on ratification behavior of countries vis-à-vis international environmental agreements (IEAs). The main argument of the paper focuses on a “democracy-civil society paradox”: although ENGOs have a positive effect on ratification of IEAs on average, this effect decreases with increasing levels of democracy. This argument is counter-intuitive and appears paradoxical because democracy is generally associated both with a more active civil society and more international cooperation. The reasons for this hypothesized effect pertain to public demand for environmental public goods provision, government incentives, and problems of collective action among ENGOs. To test the net effect of ENGOs on countries' ratification behavior, the paper uses a new dataset on ENGOs in the time-period 1973–2006. The results offer strong support for the presumed democracy–civil society paradox.