La0.8Sr0.2Ga0.8Mg0.2O3-delta (LSGM8282), La0.8Sr0.2Ga0.8Mg0.15Co0.05O3-delta (LSGMC5) and La0.8Sr0.2Ga0.8Mg0.115Co0.085O3-delta (LSGMC8.5) were prepared using a conventional solid-state reaction. Electrical conductivities and electronic conductivities of the samples were measured using four-probe impedance spectrometry, four-probe dc polarization and Hebb-Wagner polarization within the temperature range of 973-1173 K. The electrical conductivities in LSGMC5 and LSGMC8.5 increased with decreasing oxygen partial pressures especially in the high (> 10(-5) atm) and low oxygen partial pressure regions (< 10(-15) arm). However, the electrical conductivity in LSGM8282 had no dependency on the oxygen partial pressure. At temperatures higher than 1073 K, P-O2 dependencies of the free electron conductivities in LSGM8282, LSGMC5 and LSGMC8.5 were about - 1/4, and P-O2 dependencies of the electron hole conductivities were about 0.25, 0.12 and 0.07, respectively. Oxygen ion conductivities in LSGMC5 and LSGMC8.5 increased with decreasing oxygen partial pressures especially in the high and low oxygen partial pressure regions, which was due to the increase in the concentration of oxygen vacancies. The change in the concentration of oxygen vacancies and the valence of cobalt with oxygen partial pressure were determined using a thermo-gravimetric technique. Both the electronic conductivity and oxygen ion conductivity in cobalt doped lanthanum gallate samples increased with increasing concentration of cobalt, suggesting that the concentration of cobalt should be optimized carefully to maintain a high electrical conductivity and close to 1 oxygen ion transference number. (c) 2007 Elsevier B.V. All rights reserved.