The A-site ordered double-perovskite oxide, YBaMn2O5+δ, has been of recent interest for possible application as an oxygen storage material. In the present study, the oxygen non-stoichiometry of YBaMn2O5+δ has been determined as a function of pO2 at 650, 700 and 750°C by Coulometric titration at near-equilibrium conditions. The results confirm that this perovskite oxide has three distinct phases on oxidation/reduction with δ≈0, 0.5 and 1. The stabilities of the YBaMn2O5+δ phases span a wide range of oxygen partial pressures (∼10−20≤pO2(atm)≤∼1) depending on temperature. The phases interconvert at higher pO2 values at higher temperatures. The partial molar free energies (ΔμO) corresponding to the oxidation of YBaMn2O5 to YBaMn2O5.5 and of YBaMn2O5.5 to YBaMn2O∼6 were determined. The value of ΔμO in both oxidation steps becomes less negative with increasing temperature. At some T and pO2 conditions, YBaMn2O5+δ is unstable with respect to decomposition to BaMnO3−δ and YMnO3. This instability is anticipated from the previous studies of the synthesis of YBaMn2O5+δ but is more apparent in the present experiments which are necessarily slow in order to achieve equilibrium with respect to the oxygen content.