This paper proposes a research design which allows for a comprehensive assessment of the effectiveness of the European citizens’ initiative (ECI) – the first EU transnational participatory tool. It provides an analytical framework based on a broader conceptualisation of the notion of “effectiveness” which goes beyond the sole legislative function of the ECI. Consequently, the paper suggests to evaluate the ECI in five different dimensions: as a dialogue and communication enhancing tool, an awareness-raising tool, a deliberative space, a citizen-activating mechanism, as well as an agenda-setting instrument. Operationalisation of the dependent variable through a set of democratic functions that the ECI might perform allows not only to evaluate its capacity to enhance the democratic legitimacy of the EU but also to identify the tool’s limitations and propose ways of improvement. The paper also suggests a qualitative method of testing the hypothesised effects of the ECI.