In order to obtain a better understanding of the stability limits of perovskite phases used in new SOFC (solid oxide fuel cell) cathodes materials, a study of the thermodynamic properties of some perovskite-type oxides based on lanthanum strontium ferrite manganite and lanthanum strontium manganite was initiated. For the determination of thermodynamic quantities, a solid state electrochemical technique has been employed. The relative partial molar free energies, enthalpies, and entropies of oxygen dissolution in the perovskite phase, as well as the partial pressures of oxygen, have been obtained in the temperature range of 1073-1273 K. The results evidence the influence of different compositional variables on the thermodynamic properties.