The crystal structure of the compound (\mathrm{MoO}{3} \cdot 1 / 2 \mathrm{H}{2} \mathrm{O}) was solved ab initio from conventional X-ray powder diffraction data and refined by the Rietveld method. This hydrate crystallizes in the monoclinic space group (P 2_{1} / m) with unit cell dimensions (a=) (9.6715(2) \lambda, b=3.70518(7) \lambda, c=7.0975(1) \lambda, \beta=102.4\left(13(1)^{\circ}\right.). and (V=248.40(1) \AA^{3}), for (Z=4 ; 137) integrated intensities obtained by an iterative method were used to generate a Patterson map from which the initial positional parameters of two independent molybdenum atoms were derived. The remaining nonhydrogen atoms were located by Fourier methods. Refinement of the complete diffraction profile parameters ( 44 parameters, 575 reflections) converged to final agreement factors (R_{\mathrm{p}}=0.088, R_{\mathrm{wp}}=0.113), and (R_{\mathrm{F}}=\mathbf{0 . 0 2 1}). The structure consists of isolated layers parallel to (001), formed by double linear rows running along ({010 \mid) and linked together through corner-sharing (\left|\mathrm{MoO}{6}\right|) and (\left{\mathrm{MoO}{5}\left(\mathrm{H}{2} \mathrm{O}\right)\right}) octahedra. A double chain is built up from strongly distorted (\left{\mathrm{MoO}{6} \mid\right.) octahedra sharing two common edges with each other, and the next one is in a similar layer, but with (\left[\mathrm{MoO}{5}\left(\mathrm{H}{2} \mathrm{O}\right)\right]) octahedra. The successive layers are held together by hydrogen bonding to form a two-dimensional network. 1994 Academic Press, Inc.