We perform dynamic Monte Carlo simulations to understand the effect of anion adsorption on CO oxidation at the Pt(111)–electrolyte (sulfuric acid) interface. Our simulations are performed on a model for CO electrooxidation, where oxygen-containing species (adsorbed OH) formed on the Pt surface reacts with adsorbed CO by a Langmuir–Hinshelwood mechanism to form CO₂. In our site-blocking model, discharged anions adsorb on fcc and hcp sites, while CO and OH occupy the atop sites. Our simulations of blank voltammograms show a disorder to order phase transition for HSO₄ adsorption on Pt(111) surfaces near 0.4 V, RHE. This transition is observed when the difference in binding energy of HSO₄ on hcp and fcc sites, Δε ≫ k_BT. Here T is the temperature and k_B is the Boltzmann constant. We attribute this transition to the formation of the sulfate ‘butterfly’ observed in experimental base voltammograms. The ordered state () is composed of antiphase anion islands separated by domain walls. OH adsorption is observed at a potential near 0.75 V along the atop sites near the domain walls. Our simulations indicate the quenching of this phase transition for relatively high rates of OH adsorption. This phenomena is also observed in experiments for small concentrations of sulfuric acid solutions. For CO oxidation stripping voltammetry in the limit of slow CO diffusion, our simulations show a prewave (slow CO oxidation) followed by a sharp peak (rapid CO oxidation). The observed features are the result of strongly correlated kinetic events, and is explained using a nucleation–growth model. In the limit of fast diffusion, CO moving rapidly on the electrode surface washes out these effects resulting in a voltammogram with no prewave.