A dynamic Monte Carlo program CARLOS was employed in studies of phase formation on electrodes. The aim was to improve the understanding of the kinetics of adsorbed CO electrochemical oxidation by identifying relationships between nucleation-and-growth models and mean-field treatments. Initially, the development of a single phase on a surface lattice was investigated. The findings were applied to model adsorbed CO oxidation, which involves changes in two phases. By using different types of fixed oxide nucleation sites and enabling CO diffusion, we made gains in the simulation of current-time transients from potential step measurements of adsorbed CO electrochemical oxidation on surface planes of single-crystal platinum.