[en] The blue-black NbOsub(x) phases (2.4 < x < 2.5) are oxidized metastable in air to colourless (partly yellowish) Nb₂O₅ modifications already at temperatures about 200 ⁰C. At further heating stepwise and monotropic phase transformations are noticed; as final product the well known stable H modification appears. Some of the 15 new metastable Nb₂O₅ modifications obtained in this way appear in series. Therewith Nb₁₂O₂₉ (orh. and mon.) changes into two each, Nb₂₂O₅₄ and Nb₄₇O₁₁₆ into four each, Nb₂₅O₆₂ into two and Nb₅₃O₁₃₂ into one Nb₂O₅ phase. During oxidation and the following phase transformations at higher temperatures both the character of single crystal and the principle of building (block structure) of the starting material are maintained. This is especially proven from the electronoptical and X-ray investigations of Nb₁₂O₂₉ (orh. and mon.) and its products obtained by oxidation and further heating. (author)