The influence of anion adsorption on the CO electrooxidation reaction on stepped Rh[n(1 1 1) × (1 1 1)] electrodes was investigated by comparing voltammetric and chronoamperometric data obtained in 0.5 M HClO4 with previously published results obtained in 0.5 M H2SO4. Compared to sulfuric acid media, complete stripping of the CO adlayer requires fewer cycles in perchloric acid and the resulting stripping voltammetry peaks are shifted to considerably lower potentials, attesting to the reduced influence of more weakly adsorbed anions. The absence of a shoulder prior to the main oxidation peak and the higher symmetry of the main peaks implies that the electrooxidation reaction in perchloric acid is not only faster than in sulfuric acid but probably also not diffusion limited, which suggests the mean field approximation as the best mathematical model for the reaction kinetics rather than nucleation and growth. The chronoamperometric transients recorded at various potentials show only a single oxidation peak with a slight tailing at longer times. Only the Rh(1 1 1) transients display tailing, which is Cottrellian in nature. The surface diffusion coefficient of CO deduced from the Cottrell plots is more than four orders of magnitude larger in perchloric than in sulfuric acid (HClO4: 1 × 10-12 <D <2 × 10-11 cm2 s-1 vs. H2SO4: 1 × 10-16 <D <8 × 10-16 cm2 s-1), which also suggests a mean field model for HClO4 rather than nucleation and growth. Apparently, specific anion adsorption on rhodium surfaces not only affects the rate, but also the dynamics of the CO electrooxidation reaction. Thus, by varying the adsorption strength of the anion, we could, in principle, influence the diffusion rate of adsorbates on the surface and, therefore, the reaction dynamics and the overall reaction rate.