The gas phase reaction of molecular oxygen and hydrogen catalyzed by a Au(3) cluster to yield H(2)O(2) was investigated theoretically using second order Z-averaged perturbation theory, with the final energies obtained with the fully size extensive completely renormalized CR-CC(2,3) coupled cluster theory. The proposed reaction mechanism is initiated by adsorption and activation of O(2) on the Au(3) cluster. Molecular hydrogen then binds to the Au(3)O(2) global minimum without an energy barrier. The reaction between the activated oxygen and hydrogen molecules proceeds through formation of hydroperoxide (HO(2)) and a hydrogen atom, which subsequently react to form the product hydrogen peroxide. All reactants, intermediates, and product remain bound to the gold cluster throughout the course of the reaction. The steps in the proposed reaction mechanism have low activation energy barriers below 15 kcalmol. The overall reaction is highly exothermic by approximately 30 kcalmol.