By introducing finite diffusion of absorbed hydrogen atoms, adsorption, desorption and reaction patterns in H2O formation on the Pt(1 1 1) surface were simulated with the kinetic Monte Carlo (KMC) method. The results of the finite diffusion model proposed, show clearly the variation of OH desorption, which has not been observed in both the infinite diffusion model and the mean field theory. The simulation results indicate that diffusion of hydrogen atoms on the surface is essential for the reaction pattern, as the total coverage is low, and OH desorption occurs above 900 K at the value of the partial pressure ratio between hydrogen and oxygen molecules, alpha = P-H2/P-O, about 0.08. (c) 2004 Elsevier B.V. All rights reserved.