We report on a combined high resolution electron energy loss spectroscopy and x-rays photoelectron spectroscopy investigation of the interaction at room temperature of H2O with thin and ultra-thin MgO films grown on Ag(100). We find a strongly enhanced dissociation probability in the monolayer and sub-monolayer regime, indicative of an active role of the Ag substrate in the dissociation process. The active sites are suggested to be oxygen atoms with low coordination at the border of monolayer MgO islands. (C) 2003 American Institute of Physics.