In this work, nanostructured Ln_xCe_1-xO_2-δ (Ln: Gd and Pr; x = 0.1 and 0.2) spheres were synthesized by microwave assisted hydrothermal homogeneous co-precipitation and their properties were characterized by synchrotron radiation XRD, X-ray absorption near-edge spectroscopy (XANES) and scanning and high-resolution electron microscopy (SEM and HRTEM). In situ XRD and XANES experiments were carried out under reducing and oxidizing conditions in order to investigate the redox behaviour of these materials. The nanostructured mixed oxide spheres were found to have a cubic crystal structure (Fm3m space group). The spheres were composed of nanoparticles with an average crystallite size of about 10 nm. The Ln_0.1Ce_0.9O_2-δ compositions exhibited the highest specific surface area (∼60 m² g^-1). In situ XRD experiments showed an increase in lattice parameters upon reduction, which was attributed to the reduction of Ce⁴⁺ and Pr⁴⁺ cations to Ce³⁺ and Pr³⁺, which have larger radii, and to the associated increase in V_O concentration. This increase in lattice parameters was considerably more pronounced for PrDC than GDC, and was explained by the considerably larger change in ionic radius for Pr upon reduction. XANES absorption experiments at the Ce and Pr L₃-edge showed that the changes observed upon reduction of the Pr-containing samples resulted mostly from the formation of Pr³⁺ rather than Ce³⁺, and supported the previously reported proposal that Pr³⁺ has a stabilizing effect on Ce⁴⁺.