The formation thermodynamics of YBaCo 4-x Zn x O 7+δ (x = 0, 1, and 3) oxides was determined by high-temperature oxide melt solution calorimetry. All of the studied oxides are thermodynamically metastable due to the tendency of cobalt to increase the oxidation state under oxidizing conditions as well as to significant bond valence sum mismatch for Ba and Y in 114-oxides. Complex phase evolution in YBaCo 4 O 7+δ at 350-400 °C upon oxygen absorption was revealed using incremental precise oxygen dosing. The calorimetric results support phase changes seen during in situ X-ray diffraction structural studies and provide high-resolution measurement of the amount and energetics of oxygen absorbed by YBaCo 4-x Zn x O 7+δ under equilibrium conditions.