Glasses based on the oxofluoroniobate anion have been characterized by vibrational and solid-state NMR spectroscopy. The mechanism of glass formation in the systems K2NbOF5-MF3 (M = Al, In) has been suggested. A glass network is built from the chains of corner-sharing octahedra through -Nb-F(O)-M- and -Nb-F-Nb- bridges. Isolated NbOF5(2-) octahedra are also present, which is consistent with the glass composition. The high ionic mobility of NbOF5(2-) due to its fast reorientations results in equalization of the Nb-O and Nb-F distances, which is reflected in the appearance of the IR band at 700-800 cm(-1) not observed in the Raman spectrum. Its assignment to bridging -Nb-O-Nb- species accepted in the literature was not proven.