The variation of the oxygen stoichiometry of CaMnO3-delta with the partial pressure of oxygen has been measured by coulometric titration at 1223, 1273 and 1373 K, and the technique used is described in some detail. CaMnO3-delta is not stable under reducing conditions; at 1273 K, CaMnO3-delta decomposes at log(Po-2/Po(2)degrees)=-1.8 with (3-delta)congruent to 2.84, under the formation of Ca2MnO4-delta and CaMn2O4. The partial molar enthalpy and entropy of oxidation of CaMnO3-delta are derived for 2.92>(3-delta)>2.86. The obtained partial molar enthalpy of oxidation is constant (H-o-H(o)degrees=-163.5 +/- 7.5 kJ mol(-1)) within the estimated uncertainty (twice the standard deviation of the mean), whereas the partial molar entropy of oxidation appears to vary with the oxygen stoichiometry. (C) 2004 Elsevier B.V. All rights reserved.