The solid-oxide electrolyte galvanic cells method has been employed in order to obtain the thermodynamic properties of some perovskite-type materials based on lanthanum strontium manganite and lanthanum strontium ferrite manganite. 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. By solid state coulometric titration, the initial composition of the perovskite phase was altered under controlled conditions. The results evidence the influence of the oxygen relative stoichiometry change on the thermodynamic properties. The effect of the iron content and of the overall A-site stoichiometry is also discussed. Broadening the potentialities of solid state EMF techniques, the method was proved very useful in thermodynamic studies when searching improved cathode materials used in solid oxide fuel cells (SOFC).