In the active interest aroused by electrochemical catalysis related to modern energy challenges, films deposited on electrodes are often preferred to homogeneous catalysts.[1] These films are, in most cases, ensembles of nanoparticles (NPs) dispersed onto a conductive network and serving as molecular or metal catalysts. Catalytic reactivity at the surface of each nanoparticle is therefore coupled with the diffusional transport of the substrate toward this catalytic site. The catalytic efficiency of the whole electrode, in terms overpotential and turnover frequency, is thus a function of the way in which the two phenomena are coupled. It has even been predicted that, when several concurrent products are formed, selectivity may be depending on this coupling, making the outcome of the competition possibly depending on the size of the NPs [2]. As a preliminary to the investigation of this emerging field we felt necessary to explore the interplay between the various modes of substrate diffusion ‒ spherical-type diffusion toward the NPs (“nanodiffusion”) against film linear diffusion and solution linear diffusion. Our main quest in this endeavor was to gather definite evidence of the very existence of nanodiffusion by singling out the conditions where it can be the sole rate-determining step of the global catalytic reaction. Our theoretical analysis of the problem indeed shows that the competition depends on a single dimensionless parameter, that contains all structural and operational parameters. These theoretical predictions are illustrated experimentally by proton reduction in aqueous medium at an electrocatalytic film composed of a mixture of platinum nanoparticles and (Vulcan) carbon powder dispersed in a Nafion film deposited on a glassy carbon electrode. The system was investigated by means of cyclic voltammetry, the density of nanoparticles and the scan rate being used as experimental variables to test the theory. Their manipulations allowed the observation, for the first time, of current-potential responses that are almost entirely governed by nanodiffusion.[3] [1] C. Di Giovanni, A. Reyes-Carmona, A. Coursier, S. Nowak, J.-M. Grenèche, H. Lecoq, L. Mouton, J. Rozière, D. Jones, J. Peron, M. Giraud, C. Tard, ACS Catal. 2016, 6, 2626. [2] C. Costentin, J.-M. Savéant, Proc. Natl. Acad. Sci. U.S.A., 2016, 113, 11756. [3] C. Costentin, C. Di Giovanni, M. Giraud, J.-M. Savéant, C. Tard., Nature Mater. 2017, 16, 1016.