[en] Triniobium hydroxide heptaoxide, Nb₃O₇(OH), was prepared hydrothermally by treating niobic acid or triniobium chloride heptaoxide with 3.0 mol/dm³ sulfuric acid at 250 - 350 ⁰C and 15 MPa. The hydroxide oxide was isomorphous with the low-pressure form of triniobium fluoride heptaoxide which is built up of blocks of the ReO₃ structure with crystallographic shear in one dimension. When heated in air, Nb₃O₇(OH) dehydrated up to 460 ⁰C to give poorly crystallized Nb₂O₅, which, on further heating, changed slowly into a less ordered precursor of M-Nb₂O₅. Hydrothermal treatment of Nb₃O₇(OH) with pure water at 400 - 500 ⁰C afforded P- and R-Nb₂O₅; the conversion of Nb₃O₇(OH) is explained in terms of the close structural relations among these three forms. (author)