Civil Society Organizations (CSOs) are considered important intermediaries between citizens and policymakers. They are assumed to function as transmission belts that filter societal preferences and channel them to policymakers. Although the ability of CSOs to connect civil society with policymakers has been put into question, it has rarely been theoretically specified and empirically tested. This paper develops a conceptualization of CSOs that examines their capacity to function as transmission belts. It does so by distinguishing two organizational dimensions related to member involvement and organizational capacity. The paper draws on a large survey of CSOs active at the EU to empirically assess these organizational dimensions and relate them to basic CSOs’ characteristics. The findings indicate that one out of three organizations approximates the ideal-type transmission belt. The findings contribute to a better understanding and assessment of CSO’s potential contribution to policy-making in representative democracies.