Gd/Sm-modified oxidation state and defect in nano-ceria are studied by X-ray photoelectron (XPS) and Raman spectroscopy technique to investigate their influence on oxy-ionic conductivity of the system. Nanosize Sm and Gd doped ceria (SDC and GDC) are synthesized in different compositions, Ce(1 - x) (Sm / Gd)(x)O3 - delta (X = 0.05 to 0.30) under identical conditions. XPS study of doped ceria system indicates enrichment of Ce3+ ions in GDC as compared to that in SDC. The quantitative analysis of Raman spectra also predicted the same trend of oxidation state of cerium. The ionic conductivity measurement by electrochemical impedance spectroscopy is demonstrated to verify the results for all compositions of dopant (x = 0.05 - 0.30). (C) 2014 Elsevier B.V. All rights reserved.