The influence of dopant size and valence state on the reduction properties, oxygen defects, and lattice strain in doped ceria has been studied fastidiously. This is of particular importance in environmental application, such as soot and CO oxidation. Herein, isovalent (Zr4+) and aliovalent (La3+, Eu3+, and Sm3+) metal ions doped ceria catalysts were synthesized by coprecipitation method with aqueous ammonia as the precipitant. The interplay of dopant size and valence in the ceria lattice is experimentally analysed in detail at the structural and electronic level by XRD, BET, TEM, Raman, TPR, and XPS techniques, and finally evaluated for soot and CO oxidation reactions. According to the analysis results, the size or the valence of the dopant or both together positively influence the ceria properties. The isovalent substituent (Zr4+), which is different in size with respect to ceria, enhanced the ceria intrinsic activity by decreasing its reduction temperature and oxygen vacancy formation energy, thereby increased the activity. Whereas, aliovalent dopants (La3+, Eu3+, and Sm3+) having different in size and valence compared to pure ceria further improved the structural and catalytic properties of bare ceria by inducing additional oxygen vacancies. The observed activity of doped materials towards soot and CO oxidation reactions is as follows: aliovalent doped ceria > isovalent doped ceria > ceria. The marked influence of isovalent and aliovalent dopants on the ceria properties are highlighted in this study. (C) 2015 Elsevier B.V. All rights reserved.