Conductivity, structure, and oxygen nonstoichiometry of compositions in the following series (100 - x) La0.6Sr0.4CoO3+/-delta center dot xCeO(2) (LSCCe) and (100 - x) La0.8Sr0.2MnO3+/-delta center dot xCeO(2) (LSMCe), where x = 0, 2, 5, and 10 mol %, are studied. CeO2 has a low solubility limit in the rhombohedral structure. Compositions containing more than 2 mol % ceria are two-phase and consist of a perovskite constituent with rhombohedral structure and ceria with cubic structure. Small additions of CeO2 influence the conductivity of the modified perovskites in a different way. The conductivity in the LSCCe series dramatically decreases with an addition of up to 10 mol % CeO2. However, the total conductivity in the LSMCe series only changes slightly. The conductivity of the perovskites as a function of oxygen partial pressure (0.21 to 1 x 10(-4) atm) is discussed. Oxygen nonstoichiometry (delta) depending on temperature and oxygen partial pressure varies in the LSCCe series in a wider range (0 <= delta <= 0.23) compared to that in the LSMCe series (0.06 <= delta < 0.14). In contrast to the LSMCe series, the compositions in the LSCCe series show a strong relationship between lattice parameters, oxygen nonstoichiometry, and conductivity. X-ray photoelectron spectroscopy indicates that distortion of Co-O bonds takes place, which could affect electronic conductivity of perovskites. (C) 2009 The Electrochemical Society. [DOI:10.1149/1.3256117] All rights reserved.