[en] Single crystals of the white MoO₃ . H₂O modification (''α-molybdic acid'') were transformed by heating to 160⁰C into perfect pseudomorphs built up from oriented MoO₃ crystallites of known structure. From the mutual orientation relationship of the unit cells of both phases involved in this topotactic reaction, as determined by X-ray photographs, a model for the so far unknown crystal structure of white MoO₃ . H₂O could be deduced. Independently, this structure was determined by X-ray diffractometer data then: space group P anti 1, a = 7.388, b = 3.700, c = 6.673 A, α = 107.8, β = 113.6, γ = 91.2⁰, Z = 2. The structure was solved from the Patterson function and refined until R = 0.088. It is built up from isolated double chains of strongly distorted [MoO₅(H₂O)]-octahedra sharing two common edges with each other. This result agrees well with the model derived from topotaxy, and it becomes evident how the MoO₃ lattice is formed through corner linking of the isolated double chains after the water molecules are removed. The study of topotactic phenomena seems rather generally applicable to deduce the main features of structures involved and for better understanding of structural relationships. (U.S.)