The first neutron diffraction study of water in silicate glasses is reported. Deuterated sodium tetrasilicate glasses, Na2Si4O9 · χD2O (χ = 5, 10 wt%), as well as a protonated glass, Na2Si4O9 · 10 wt%H2O, were prepared in rapid-quench high pressure autoclaves. Infrared (IR) measurements show, in accordance with results for other proton-containing glasses, the presence of both molecular H2O and OH species. Line shape analysis of the first sharp diffraction peak (FSDP) indicates a gradual decrease of the medium-range order with increasing total water (H2O + OH) content. A structural mechanism explaining the observed changes in the FSDP is proposed. Combined least-squares fits, using Gaussian functions, of the first SiO, NaO and OO peaks in the corresponding total pair correlation functions, T(r), indicate that the short-range order of the silicate network is not affected by the presence of water. However, the root mean square displacements of the Na atoms decrease, suggesting that the NaO coordination becomes more ordered in hydrous silicate glasses. The first-difference of the correlation functions, D(r), for the glasses with 10 wt% total water content is obtained. The OH contributions are identified and indicate, in accordance with the analysis of the FSDP, that the OH groups lead to significant changes in the silicate network on the 3–5 A scale. The average OH distance determined from the total pair correlation functions is 0.99 A. Three types of hydrous species (H2O and / or OH), participating in hydrogen-bonding, are observed in the IR spectra with average OH...O bond lenghts, equal to 3.0, 2.65 and 2.55 A, respectively.