Neutron diffraction studies are reported on H2O at temperatures of 300 and 400 degrees C. The method of isotopic substitution is applied to three mixtures of H2O and D2O, and the diffraction data are used to determine the three radial distribution functions g(HH)(r), g(OH)(r), and g(OO)(r) These results can be used to-discuss changes in nearest neighbor structure between water molecules, and to assess the degree of usefulness of representative (usually pairwise) model potentials.