The solid-oxide electrolyte galvanic cell method has been employed in order to characterize, from the thermodynamic point of view, some materials based on substituted lanthanum chromites: La1-xMxCrO3 (where M = Sr, Ca; x = 0.2, 0.3) and La0.8Sr0.2Cr0.95Mg0.05O3. The thermodynamic properties represented by 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 as a function of temperature and substituent nature within the temperature range of 1073-1273 K and in reducing atmosphere (10(-5) Pa). The influence of oxygen stoichiometry change on thermodynamic properties was examined using the data obtained by a coulometric titration technique coupled with EMF measurements. An attempt to verify a previously proposed defect-structure model is also made.