This paper explores the role of mesoscopic mass transport in controlling electrocatalytic selectivity, a phenomenon often overlooked in favor of atomic-level active site considerations. The authors introduce the desorption–re-adsorption–reaction mechanism, which involves the exchange of surface-bound reaction intermediates between the electrode and bulk electrolyte. This mechanism introduces kinetic competition that can significantly influence selectivity, particularly in technologically important reactions like CO2 reduction on Cu-based catalysts. By combining microkinetic and transport modeling, the authors demonstrate how catalyst morphology, such as loading and particle shape, affects the probability of re-adsorption of intermediates, thereby altering selectivity. The model accurately reproduces experimental trends and highlights the importance of surface roughness as a descriptor of catalyst morphology across multiple length scales. The findings provide a complementary explanation to traditional active site models and offer insights into catalyst design and degradation, emphasizing the need for careful consideration of mass transport in electrocatalytic processes.This paper explores the role of mesoscopic mass transport in controlling electrocatalytic selectivity, a phenomenon often overlooked in favor of atomic-level active site considerations. The authors introduce the desorption–re-adsorption–reaction mechanism, which involves the exchange of surface-bound reaction intermediates between the electrode and bulk electrolyte. This mechanism introduces kinetic competition that can significantly influence selectivity, particularly in technologically important reactions like CO2 reduction on Cu-based catalysts. By combining microkinetic and transport modeling, the authors demonstrate how catalyst morphology, such as loading and particle shape, affects the probability of re-adsorption of intermediates, thereby altering selectivity. The model accurately reproduces experimental trends and highlights the importance of surface roughness as a descriptor of catalyst morphology across multiple length scales. The findings provide a complementary explanation to traditional active site models and offer insights into catalyst design and degradation, emphasizing the need for careful consideration of mass transport in electrocatalytic processes.