The enthalpies of oxidation of CaMnO3 - δ, Ca2MnO4 - δ, and SrMnO3 - δ have been determined by in situ oxidation in a high-temperature adiabatic calorimeter. CaMnO3, Ca2MnO4, and SrMnO3 were synthesized by the EDTA precursor or the ceramic method, and partly reduced materials were prepared by topotactic reduction in diluted H2 (5−10%) at 290−355 °C. Unit cell dimensions and oxygen stoichiometry of partly reduced and reoxidized materials were determined by powder X-ray diffraction, thermogravimetry, and iodometric titration. The enthalpy of oxidation of SrMnO3 - δ was considerably less exothermic than the corresponding value for CaMnO3 - δ, and the enthalpy of oxidation of Ca2MnO4 - δ was more exothermic than for CaMnO3 - δ. The enthalpy of oxidation is discussed in terms of the basicity of oxides, the Goldschmidt tolerance factor, and the crystal structures of the oxidized and reduced materials. Finally, based on the enthalpy of oxidation, the oxygen defect chemistry of these materials is estimated using simple thermodynamic models. Using the measured enthalpy of oxidation and estimated entropy of oxidation, the measured oxygen nonstoichiometry is reproduced rather well. A free fit of the same models to the experimental data gives enthalpies of oxidation that are far more exothermic than the measured values. The present findings demonstrate that simple models based on point defect equilibrium may give significant errors in the enthalpy and entropy of formation of point defects, despite a good fit to the free energy.