Using microkinetic modeling, we have determined the closed catalytic cycle that describes the water gas shift reaction on copper catalysts. Eight elementary reactions constitute the cycle. Dissociation of water and the recombination of surface hydrogen result in dihydrogen. While surface hydroxyl reacts with surface CO to give a surface carboxyl intermediate, which, in turn, reacts with another surface hydroxyl to give water and carbon dioxide to complete the cycle. The kinetic model further predicts that the most abundant surface species is a formate species formed via the hydrogenation of carbon dioxide. This reaction is not part of the catalytic cycle, and surface formate is a spectator species which increases substantially at high pressure and low temperature. Adsorption-desorption of dihydrogen is equilibrated; while water dissociation is kinetically significant with a high degree of irreversibility. And, depending on the reaction conditions, carboxyl formation also becomes kinetically important. The model identifies the binding energies of surface H, HCOO, and OH to be important parameters and allows us to explain the difference in the site time yields of various copper catalysts. (C) 2011 Elsevier Inc. All rights reserved.