The interaction of water with MgO (100) single crystal surfaces cleaved in situ has been studied by low energy electron diffraction and helium atom scattering in the temperature range 80 K-230 K. At T-crystal=100-180 K water forms a layer with a c(4X2) symmetry in good agreement with previous spot profile analysis of low energy electron diffraction experiments. Adsorption at T-crystal=185-221 K leads to the formation of a new ordered phase. The results of the low energy electron diffraction and elastic helium atom scattering experiments show that this high-temperature phase has a (3X2) symmetry, and that the unit cell contains a glide plane. The isosteric heat of adsorption at half coverage Q(st)=85.3 kJ/mol has been determined from equilibrium adsorption isotherms measured between 210 and 221 K.