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The oxygen content in CaMnO3–δ is studied by a coulometric titration technique depending on temperature and oxygen partial pressure variations in the ambient atmosphere. The δ–T phase diagram is derived from the obtained data where single-phase fields are outlined for orthorhombic, tetragonal, and cubic structures. The thermal expansion coefficient considerably larger in the cubic phase than in the tetragonal one is related with formation of large Mn3+ cations at depletion of oxygen from the crystalline lattice. Negative thermopower is explained by concomitant reactions of oxygen loss and charge disproportionation, $$ {\hbox{2M}}{{\hbox{n}}^{{{4 + }}}}{\hbox{ = M}}{{\hbox{n}}^{{{3 + }}}}{\hbox{ + M}}{{\hbox{n}}^{{{5 + }}}}{ } $$ . The forbidden energy gap in CaMnO3–δ is evaluated to be about 0.5 eV.