Vibrational spectroscopy has been used to study the interaction at room temperature of H2O with ultrathin alkali films grown on Cu(1 1 1), Ni(1 1 1) and Ag/Ni(1 1 1). We find strongly enhanced dissociation efficiency in the monolayer regime of Na/Cu(1 1 1), indicative of an active role of the existing Na-derived quantum wells in hydrolysis. No water adsorption has been observed for alkalis (Na, K) adsorbed on Ni(1 1 1), very likely due to the absence of electron confinement in these systems. On the other hand, partial water dissociation has been revealed in K-doped silver thin layers on Ni(1 1 1), in the presence of Ag 5sp-derived quantum wells.