The phase diagram of the Lu2O3-LuF3 binary system was investigated by quantitative differential thermal analysis in the (1000-1750) K temperature range, as well as by X-ray powder diffraction analysis. At temperatures lower than 1075 K, only the well-known compounds Lu2O3, LuOF, LuF3 and the series of fluorite-related orthorhombic vernier phases of the general composition LunOn-1Fn+2 with n = 5-10 were found to exist. At (1076 +/- 4) K an anion-deficient, fluorite-related and presumably monoclinic phase of the composition Lu(O,F)(1.91+/-0.01) is formed by eutectoid reaction of Lu2O3 and LuOF. LuOF decomposes at (1079 +/- 4) K in a peritectoid reaction into Lu(O,F)(1.91+/-0.01) and the vernier phase Lu10O9F12. All vernier phases were characterized with respect to their lattice parameters, and it was found that the lattice parameters depend linearly on composition.