The model has been proposed to simulate numerically the reaction O + CO -> CO2 and occurring phase transitions on Pd(1 1 1) surface. We calculate the phase diagram for this system by kinetic Monte Carlo method. It shows the phase transitions p(2 x 2)(O) -> root 3 x root 3R30(O)degrees and p(2 x 2)(O) -> root 3 x root 3R30(O)degrees -> p(2 x 1)(O) with increase of CO coverage for room and intermediate temperatures, respectively, while in the low temperature limit the direct p(2 x 2)(O) -> p(2 x 1)(O) phase transition is observed. We demonstrate that the reaction rate is the crucial factor determining the occurrence of the p(2 x 1)(O) phase and vanishing of the root 3 x root 3R30(O)degrees with decrease of temperature. The results indicate that the reaction proceeds inside both the p(2 x 2)(O) and root 3 x root 3R30(O)degrees phases, but on the perimeter of the domains of p(2 x 1)(O) structure. (c) 2006 Elsevier B.V. All rights reserved.