The European Economic and Social Committee (EESC) and the Committee of the Regions (CoR) are two advisory bodies of the European Union involved in a broad array of policy areas. However, little is known about the effects of the two committees on the positions of the other institutions or final policy outcomes. This article investigates to what extent and under what conditions the CoR and the EESC can exert influence. Based on a comprehensive survey conducted in 2010, a series of hypotheses derived from a neo-institutionalism framework are tested. It is found that consultative committees are not very influential overall. Nevertheless, they can exert influence under certain scope conditions, including the speed with which they produce recommendations, the quality of the recommendations and the resonance with the addressees' prior beliefs.