Cluster models and the atom superposition and electron delocalization molecular orbital (ASED-MO) band shift technique have been used to study the electrochemical potential dependence of H2O(ads) decomposition to OH(ads) and H(ads) on Pt(111) and (100) anodes. The water molecule is found to bind most stably to the 1-fold site of the (111) surface and the 2-fold bridging site of the (100) surface. The transition state structure is closer to the 1-fold structure, which explains our result of lower activation energy over the Pt(lll) surface. This finding provides an interpretation for results of electrochemical measurements in the literature.