This paper sheds new light on deliberative dynamics at work in comitology. Starting from the findings of a previous research on the frequency of consensus within the executive committees of the EU in 2008, which unveiled the existence of contested votes in over a quarter of member states representatives’ deliberations, this paper seeks to provide a measure of the intensity of opposition to Commission’s draft implementing proposals before and after the post-Lisbon reform of comitology. Applying this approach to data from the comitology register (2008-2013), the paper analyses the structure of contestation, proposes an index of opposition and tests the factors that may explain variations. Attention is paid to rationalist and sociological arguments, policy types and procedural settings before and after the last reform of the comitology system. Our findings reveal that conflict with Commission’s positions, although weak, is not that rare during deliberations prior to the formulation of an opinion. The paper is organized as follows. The first part is theoretically grounded in scholar’s debates concerning the nature of interactions in EU executive committees. The second part presents the data and method of the research with particular emphasis on the index of opposition. The third part exposes the patterns of support and opposition to Commission’s implementing proposals as revealed by the empirical study. The fourth part attempts to explain these findings by testing the main hypotheses that have been traditionally formulated regarding Member states-Commission deliberative dynamics. The paper ends with some remarks on the strength of dissent in comitology.