Crystal structure, thermal expansion, oxygen stoichiometry, thermoelectricity, and electrical conductivity of compositions in the system La1-xSrxCo0.2Fe0.8O3 with 0 less than or equal to x less than or equal to 0.6 were studied as function of temperature and Sr content, in air. The solubility of Sr in the sintered perovskite-type oxide (ABO(3)) was limited to x less than or equal to 0.4. The observed p-type electrical conduction appeared to occur via a small-polaron hopping mechanism. The thermally-induced oxygen loss caused a lattice expansion plus decreases in both the carrier concentration and the carrier mobility. A semi-empirical model was developed which takes into account the thermally activated disproportionation of Co3+ ions into Co4+ and Co2+ pairs, and the ionic compensation of oxygen vacancies formed at high temperatures. The concentrations of B-site ions (Co or Fe) in different valence states were calculated using this model and experimental data.