In this study, we demonstrate the role of mesoscopic mass transport in determining electrocatalytic selectivity for a number of technologically important processes, including the eORR and different products of eCO 2 RR on Cu. Quantitative understanding of the under-lying "desorption-re-adsorption-reaction" mechanism is established, for the first time to our knowledge, by developing a simple multi-scale model that couples diffusion to the electrochemical surface kinetics. Our model correctly reproduces a series of trends found in the experimental literature, while providing an alternative or, at least, complementary explanation to changes in electrocatalytic selectivity beyond the active site model. Within this picture, the electrode surface roughness emerges as a descriptor of selectivity that effectively captures the influence of catalyst morphology across multiple length-scales: the mesoscopic scale at the inter-particle level, microscopic scale at the intra-particle level, and nanoscopic scale at the atomic level. Our analysis finally highlights the relevance of the "desorption-readsorption-reaction" mechanism in catalyst degradation as morphological changes over time (e.g. due to nanoparticle agglomeration) induce corresponding changes in electrocatalytic selectivity.