Phase equilibria in the molten system KF-K2NbF7-Nb2O5 were investigated up to 20mol% of Nb2O5 by thermal analysis and differential scanning calorimetry. In the binary and ternary systems oxofluoroniobate compounds are formed. They were evidenced by comparison of experimetal X-ray diffraction patterns with those in the international X-ray diffraction database (PDF-2). In the experimental compostion range the phase diagram of the molten KF-K2NbF7-Nb2O5 system consists of five crystallisation fields corresponding to KF, K3NbF8, K2NbF7, K2NbO3F and K3NbOF6.