Crystal structure, temperature dependence of molar volume and optical property of single phase BaCe1-xRExO3-delta were investigated. The crystal structure of BaCe1-xRExO3-delta (RE; Yb, Y, Eu, and Sm) and BaCe1-xRExO3-delta changes from primitive orthorhombic to monoclinic and body-centered orthorhombic, respectively, by increasing x. Almost linear relationship between ionic radius of RP3+ and the molar volume are observed for BaCe1-xRExO3-delta with RE of Yb, Y, Eu, Sm; however, that of BaCe1-xRExO3-delta is smaller than the relationship. As one of the origins, proposed is that Nd in BaCe(1-x)RExO(3-delta) is near 4+, showing agreement with reported oxygen nonstoichiometry, in-situ x-ray absorption spectra and reduction expansion observed in this work. As another origin, partial Ba-site mixing of Nd3+ is proposed from separation of BaCO3 from BaCe(0.9)Nc(0.1)O(3-delta) by high temperature sintering. The lower optical band gap of BaCe(0.9)Nc(0.1)O(3-delta) observed by reflectance spectroscopy, canals be explained by the partial mixing. (C) 2016 Elsevier Ltd. All rights reserved.