Abstract Phase equilibria in the systems, Dy2O3–V2O3–V2O5 and Ho2O3–V2O3–V2O5, were established at 1200 °C by changing the oxygen partial pressure from 4.32 to –7.50 Pa in terms of logPo2. In both systems, Ln2O3, LnVO4, LnVO3, VnO2n−1(n=2–7), VO2, and Ln8V2O17(4Ln2O3·V2O5) are stable, and compounds with mole ratio Ln2O3/V2O5=5⁄1 are not stable under the present experimental conditions. On the basis of the established phase diagrams, the standard Gibbs energies of reactions, (1) 3 Dy2O3+2 DyVO3+O2=Dy8V2O17, (2) DyVO3+1/2 O2=DyVO4, (3) 3 Ho2O3+2 HoVO3+O2=Ho8V2O17, and (4) HoVO3+1/2 O2=HoVO4, were determined to be −278, −123.8, −277, and −122.4 kJ, respectively. LnVO4, Ln8V2O17, V2O3, and VO2 have nonstoichiometric compositions. ΔG° values for reactions (2) and (4) fit well on the linear line presented by us.