Substituted ceria is a material which has attracted great interest in solid oxide fuel cell technology [45]. Due to its chemical stability, it may also be a promising candidate as a mixed conductive membrane for oxygen separation with flue gas contact. As part of this paper, dilatometry measurements and X-ray powder diffraction experiments were carried out on ceramic materials in air and in argon with two separate series involving the substitution of ceria; cerium was substituted with different amounts of Gd and Pr. The first substitution of Gd caused an increase of the unit cell with a small rise in linear thermal expansion in relation to pure ceria. CeO2 (-) (delta) Band Ce0.8Gd0.2O2 - delta showed no chemical expansion in both atmospheres. The second substitution of Gd with Pr decreased the unit cell in relation to pure ceria. It is therefore understandable that Pr is preferentially introduced and shown in this work as Pr4+ in ceria. These samples showed a remarkable chemical expansion in air and in argon. The chemical expansion displayed a clear positive correlation with increasing Pr content. The thermal expansion coefficients are comparable to the first substitution without any trend in relation to the amount of Pr. The dilatometry behaviour above 400 degrees C can be explained due to the release of oxygen and simultaneous reduction of Pr4+, forming Pr3+ and oxygen vacancies. The results are in accordance with the two competing processes of forming vacancies (lattice contraction) and ionic radius change (lattice expansion) [16,27]. After cooling in air, the samples showed no residual expansion. In contrast, these samples displayed a remarkable residual expansion in argon of about 0.87% of the total relative expansion of 1.85% for Ce0.8Pr0.2O2 - delta. (C) 2015 Elsevier B.V. All rights reserved.