The high alkali concentration regions of the phase diagrams for the system Na2O-Al2O3-H2O were studied at 95 degreesC and 110 degreesC, to provide a chemical database for producing monosodium aluminate hydrate (Na(2)O(.)Al(2)O(3)(.)2.5H(2)O). In the phase diagram at 95 degreesC, the solid phases of 4Na(2)O(.)Al(2)O(3)(.)12H(2)O and 6Na(2)O(.)Al(2)O(3)(.)12H(2)O, besides (Na2OH2O)-H-., Al(2)O(3)(.)3H(2)O, and Na(2)O(.)Al(2)O(3)(.)2.5H(2)O, were discovered. In the phase diagram at 110 degreesC, 4Na(2)O(.)Al(2)O(3)(.)12H(2)O was found missing. These phase diagrams show that all these sodium aluminate hydrates decompose in the presence of water and that the higher the temperature, the larger the region with Na(2)O(.)Al(2)O(3)(.)2.5H(2)O as the equilibrium solid.