The reaction between Na and multilayer water on MgO(100)/Mo(100) has been studied by metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS), and temperature-programmed desorption (TPD). A water multilayer was exposed to Na at 100 K. During exposure, an attenuation of the surface water concentration concomitant with the formation of surface hydroxyls and metallic Na clusters is observed. After an anneal to 155 K, the metallic Na clusters disappear and the surface hydroxyls are essentially removed from the outermost surface, leaving water as the dominant surface species. Further annealing to >170 K leads to massive water and NaOH desorption, as indicated by TPD. After desorption of all multilayer components the species stabilized at the surface is primarily hydrated NaOH, which is stable up to 530 K.