Perovskite oxides BaCe1-xEuxO3-delta (x = 0.05, 0.10, 0.15, and 0.20) were prepared using starting ratios of Ba/(Ce + Eu) = 1.0 and 1.1. X-ray diffraction analysis shows that preparations with a slight initial barium excess crystallized as a single perovskite phase, while the stoichiometric preparations crystallized with a trace of CeO2 due to Ba loss. EPR measurements indicate that the samples have no Ba vacancies and that Eu and Ce ions are in trivalent and tetravalent states, respectively. The single-phase samples synthesized from Ba/(Ce + Eu) = 1.1 preparations are shown to have different cell volume trends versus x, lower average thermal expansion coefficients, and higher proton conductivities in water-containing atmospheres between 80 and 700 degreesC, compared to samples synthesized from the Ba/(Ce + Eu) = 1.0 preparations. These observations are explained in terms of the impurity CeO2) phase, Eu doping-site preferences, and oxygen vacancy generation. (C) 2004 Elsevier B.V. All rights reserved.