The adsorption of water on well-ordered MgO(100) surfaces has been studied using metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS), and temperature-programmed desorption (TPD). Experimental evidence is presented that water adsorbs dissociatively and molecularly within the first monolayer on well-ordered MgO(100) surfaces. These results support recent theoretical predictions [Giordano et al. Phys. Rev. Lett. 1998, 81, 1271] that water partially dissociates on MgO(100) surfaces.