In this work, we build a microkinetic (MK) model of the CO electro-oxidation reaction using density functional theory (DFT) calculations and scaling relations to understand the interplay between bifunctional mechanisms and electronic modification. A single-site model is first used to identify trends in activity under different operating conditions. The model is then extended to include two site types to understand the surface characteristics necessary for bifunctional activity. For these general cases, we show that the "optimal" adsorption energetics change as a function of the applied potential. We then perform a case study by explicitly modeling both site types in platinum-ruthenium alloy models to assess whether bifunctional or electronic effects are most operative. We find that if these materials indeed remain metallic alloys, then electronic effects are likely dominant, with the sole active sites being electronically-modified Pt sites. Ru sites are spectators that favorably perturb the electronic structure of Pt. (C) 2020 Elsevier Inc. All rights reserved.