Theoretical electronic structure calculations are reported for the photoinduced dissociation of water adsorbed on a 31-atom silver cluster, Ag31 to produce H2 and adsorbed O.Both ground and excited electronic state processes are considered.The excited electronic state of interest is formed by photoemission of an electron from the Ag cluster and transient attachment of the electron to the adsorbed water molecule.A very large energy barrier for H2 formation (3.51 eV) is found for the ground state process while the barrier in the excited state is much smaller (0.47 eV).