The phase-equilibrium diagram for the system Nb2O5-B2O3 was determined experimentally using the quenching technique and examining the samples by optical microscopy and x-ray powder diffractometry. The system contains one binary compound of approximate composition "3Nb2O5 · B2O3," which melts incongruently at about 1150 °C to H -Nb2O5 and B2O3-rich liquid. A large region of liquid immiscibility extends, at 1352 °C, from 10 mole percent Nb2O5 to 65.7 mole percent Nb2O5. The theoretically calculated extent of immiscibility is in reasonable agreement with the experimentally determined value.