[en] Uniform Ce_1-xZr_xO₂ (x=0.2-0.8) nanocrystals with ultra-small size were synthesized through a thermolysis process, facilitated by the initial formation of precursor (hydrated (Ce,Zr)-hydroxides) at low temperature. TEM, XRD, EDAX, and Raman spectra were employed to study the formation of the solid solutions with various Ce/Zr ratios. Ultraviolet-visible (UV-vis) spectra showed that the ratios of Ce³⁺ to Ce⁴⁺ in both surface and bulk for the as-prepared Ce_1-xZr_xO₂ nanocrystals increased with the zirconium content x. The well-distributed Zr and Ce in the hydrated (Ce,Zr)-hydroxides before their thermolysis became the crucial factor for the structural homogeneity of the products. In addition, this strategy was extended to the synthesis of Ce_1-xGd_xO_1-x/2, Ce_1-xSm_xO_1-x/2, and Ce_1-xSn_xO₂ solid solutions. Catalytic measurements indicated that the ceria-based catalysts were active for CO oxidation at temperatures beyond 250 deg. C and the sequence of catalytic activity was Ce_0.5Zr_0.5O₂>Ce_0.8Zr_0.2O₂>Ce_0.2Zr_0.8O₂>Ce_0.5Sm_0.5O_1.75. - Abstract: Uniform ultra-small nanostructured Ce_1-xZr_xO₂, Ce_1-xGd_xO_1-x/2, Ce_1-xSm_xO_1-x/2, and Ce_1-xSn_xO₂ solid solutions with homogeneous textures were synthesized through a thermolysis process, facilitated by the initial formation of precursors (hydrated (Ce,M)-hydroxides). Display Omitted