We have used a Holzafpel-type diamond anvil cell to determine the NH3-H2O phase diagram in the region from 0 to 33 mol % NH3, 240 to 370 K, and 0 to 5 GPa. The following phases were identified: liquid; water ices Ih, III, V, VI, VII, and VIII; ammonia monohydrate, NH3⋅H2O; and ammonia dihydrate, NH3⋅2H2O. Ammonia dihydrate becomes prominent at moderate pressures (less than 1 GPa), with planetologically significant implications, including the possibility of layering in Titan's magma ocean. ¿American Geophysical Union 1987