The structures of the KNb4O5F (I) (synthesized for the first time) and the NbO2 (II) phases formed together with NbO during decomposition of reduced oxofluoroniobate K2-xNb4O3(O,F)(3)F (III) in the (NaCl + KCl) melt are studied by X-ray methods. The tetragonal structures of compounds I and II are refined up to R = 0.06 and R = 0.0186, respectively. The unit-cell parameters are determined as a = 4.155(3) and c = 8.377(6) Angstrom, sp. gr. P4/mmm for compound I, a = 13.698(3) and c = 5.976(1) Angstrom, sp. gr. 14(1)/a for compound II, and a = 4.151(3) and c = 21.64(2) Angstrom, sp. gr. P4/mmm for compound III. Decomposition of compound III is considered in terms of different stabilities of its structural modules (the two-dimensional periodic NbO and the KF fragments). The formation of NbO and compound I is associated with the decomposition of KF fragments and the preservation of NbO fragments. In compound I, only one of two KF networks (similar to those separating the NbO fragments in compound III) is preserved. In NbO, two-dimensional condensed NbO fragments form a three-dimensional framework. Compound II has neither of the two modules observed in III, but the NbO4 squares forming an NbO fragment are preserved. The defects of the real structure of compound I, which give rise to the formation of a one-dimensionally disordered KNb4O5F structure, are analyzed. The discrepancies between the structural characteristics of compound II obtained in the present study and reported earlier are discussed. [References: 9]