The electrochemical oxidation of saturated and sub-saturated CO adlayers on Pt(111) in 0.5 M H2SO4 has been studied using chronoamperometry. For the saturated CO coverage the oxidation is initiated by an apparently zeroth-order process of removing 2-3% of the adlayer, followed by the main oxidation process, which is shown to be of the Langmuir-Hinshelwood type with a competitive adsorption of the two reactants, CO and OH. The Langmuir-Hinshelwood kinetics can be modeled using the mean-field approximation, which implies fast diffusion of adsorbed CO on the Pt(111) surface under electrochemical conditions. The apparent rate constant for the electrochemical CO oxidation and its potential dependence are determined by a fitting of the experimental data with the mean-field model. For sub-saturated CO coverages the overall picture is shown to be more complicated and remains to be understood.