The interest in thin and ultrathin oxide films is increasing rapidly due to the new properties and the many possible technological applications of such materials. In this frame the interaction with water, a major constituent of our atmosphere, is an essential issue for a better characterization of oxide film-based devices. We report here a detailed high-resolution electron energy loss spectroscopy and X-ray photoelectron spectroscopy study of the reactivity of ultrathin MgO films grown on Ag(100) toward H2O. We find that only OH groups are detected at 3 10 K, while at low temperature molecular adsorption prevails. In the former case we observe a strongly enhanced dissociation probability for monolayer and submonolayer MgO films, indicative of an active role of the Ag substrate in the dissociation process. The active sites are suggested to be low-coordinated ions at the border of monolayer MgO islands. Aging phenomena, previously observed on MgO ultrathin films,(1) have been confirmed. Although their origin could not be definitively determined, our data strongly suggest water adsorption not to be the major cause of this process.