Ce1 − x Ln x O2 − d (Ln = Sm, Pr; x = 0–0.30) oxides have been prepared through pyrolysis of polymer-salt systems, and their particle size and specific surface area have been determined. Their sintering behavior and thermal expansion have been studied using dilatometry. The thermal expansion of the Pr-containing materials is shown to vary considerably with temperature. X-ray diffraction data (obtained at various temperatures and oxygen pressures) suggest that the Ce1 − x Pr x O2 − d oxides undergo a low-temperature phase transition, presumably a reversible decomposition of one solid solution to two others. The structural parameters of the phases involved and the oxygen nonstoichiometry of the samples are determined.