Partial electronic and oxide ionic conductivity in Co doped LaGaO3 was investigated with a polarization method and it was seen that the partial electronic conductivity became significant with increasing temperature. P-O2 dependences of the partial electron and hole conductivities at 1173 K are almost proportional to P-O2(-1/4) and P-O2(1/4), respectively, in accordance to Wagner's theory. On the other hand, P-O2 dependences of electron and hole conductivities deviate from those of Wagner's theory and became much smaller as temperature decrease. The small P-O2 dependence of the partial electronic conductivity could be explained by extrinsic electrons or holes originating from oxidation or reduction of cobalt. In order to explain the small P-O2 dependence, average valence numbers of Co were measured by redox titration. It was seen that the P-O2 dependence of the Co valence number is similar to those of partial electron and hole conduction. Consequently. the small P-O2 dependence of partial electronic conduction could be assigned to the small P-O2 dependence of the valence number of Co. (C) 2004 Elsevier B.V. All rights reserved.