The "desorption-re-adsorption-reaction" mechanism describes the fate of a specific surfacebound, yet volatile, intermediate along the reaction path. The mechanism is schematically illustrated in Fig. where this key intermediate X * (superscript * denotes an adsorbed configuration) faces a branching in the reaction path to either (i) continue along the potentialdependent surface route towards full catalytic conversion, or (ii) desorb above the surface where it starts to build up a local concentration gradient. In the second case, if diffusion is fast enough, the near-surface species X ( * ) will leave the surface entirely and be detected as an early partially-converted product X(aq) in the bulk (aqueous) solvent. If diffusion is relatively slow, however, X ( * ) faces the prospect of re-adsorbing onto the surface where it will once again be subjected to the (i) vs (ii) competition described above. The mechanism thus essentially boils down to a competition between surface kinetics and diffusion. To describe this competition, we couple these two key components within a simple kinetic model of the selectivity-determining step. A surface-bound, closed shell intermediate X * faces the kinetic competition between a forward surface redox step vs. desorption. Following desorption, if diffusion is fast enough, near-surface X ( * ) species will escape into the bulk electrolyte and be detected as an early, partially-converted product. Our model couples diffusion with the surface kinetics via the surface roughness ρ which effectively captures the influence of catalyst morphology across multiple length-scales: at the inter-particle (via e.g. catalyst loading), intra-particle (via e.g. microscopic surface roughening), and atomic (via alloying) levels. Additionally shown are the other six parameters entering our model: free-energy barriers (∆G ‡ rdx (U ), ∆G ‡ des , ∆G ‡ ads ), as well as diffusion-related properties (D diff , L diff , H s ) as described in the text.