The oxygen effect on niobium complexation in NaCl-KCl-K2NbF7 melts has been studied at 700-720°C by means of linear sweep voltammetry, potentiostatic electrolyses, X-ray diffraction, IR-spectroscopy and optic crystallography analyses. The NaCl-KCl-K2NbOF6, NaCl-KCl-K2Nb02F4 and NaCl-KCl- K2NbF7 melts with addition of Na02 and Nb205 as source of oxygen were investigated. It was shown that at a molar ratio O/Nb=l the predominating complexes in the melt is NbOF5Cl3-. The reduction of these oxo complexes occurred at potentials more cathodic than those for oxygenless complexes. Increase in the O/Nb molar ratio in excess of one leads to a decrease of niobium concentration in the melt. The mechanism of the solubility of K3Nb02F4 compound in NaCl-KCl melt was determined.