Numerical simulations of the voltammetric electrooxidation of CO adsorbed on Pt(111) surfaces are presented. For CO dosage, the numerical results confirm previous proposals, based upon current experimental information, concerning the CO distribution on the Pt surface. Specifically, whereas at low coverages (below 1/3) CO forms easily oxidizable open structures, at high coverages compact domains hinder the oxidation process. The two peaks observed in the voltammetric profiles are thus a consequence of topological differences between the CO distributions at low and high coverages. We also present new experiments on the electrooxidation of CO adlayers on Pt(111) formed upon the dissociative adsorption of formic acid. In this case our results indicate that a single oxidation peak remains up to coverages near 2/3. This striking new result is also reproduced in our simulations by taking the same kinetic parameters as in the previous case, and assuming that the CO are distributed on a honeycomb lattice, an assumption consistent with the characteristics of the dissociative adsorption of formic acid.