The compositions of anion‐deficient zirconia and thoria in equilibrium with O2were measured from 1 to 10−6atm and 1400° to 1900°C; for ZrO2‐x(po2in atm, and T in °K), log x∼−0.890‐[(0.400×104)/T]‐[(logp)/6]; for ThO2‐x, log x∼−1.870‐[(0.340×104)/T]‐[(logp)/6]. The ZrO2‐x‐Zr boundary was located at x=0.014 at 1800°C; thoria was single‐phase over the entire range. Consistent results were obtained when O2/inert gas mixtures were used, but use of H2/H2O and CO/CO2at 1000° to 1200°C gave abnormal and, in the latter case, erratic data; side reactions in these atmospheres are inferred. The monoclinic‐tetragonal phase change of ZrO2and the lattice thermal expansion, room‐temperature Young's modulus, and strength properties of ZrO2and ThO2bodies were not appreciably altered by oxygen deficiency. The lattice dimensions decreased s