The coadsorption of D2O and Na on the MgO(100)/Mo(100) surface has been studied by metastable impact electron spectroscopy and ultraviolet photoelectron spectroscopy (He I). The initial layer of D2O adsorbed on the MgO(100) surface at 100 K "wets" the surface. For multilayer adsorption, the outermost water molecules exhibit an electronic structure which is very similar to gas phase water. Na dosed onto a D2O-precovered MgO surface leads to the formation of a hydroxide species, most likely NaOH. This hydroxide species is stable to 530 K.