The effects of non-reducible dopants on copper-ceria catalysts for the preferential oxidation of carbon monoxide (CO PROX) were experimentally investigated by adding a non-reducible rare-earth element into the ceria support. Gadolinium-doped cerium oxides were synthesized by a combustion method as a support material for the copper-ceria catalytic system. Various compositions of the catalysts, i.e., CuO/Ce1-xGdxO2-delta (x = 0, 0.05, 0.1, 0.13, 0.18, 0.25 and 0.35), were prepared. The physical, structural, redox and surface chemical properties of the prepared catalysts were characterized by inductively coupled plasma mass spectrometry (ICP-MS), N-2 isotherms, X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), H-2 temperature-programmed reduction (H-2-TPR) and X-ray photoelectron spectroscopy (XPS) analyses. To investigate the influence of the dopants on the CO PROX, an activity test was conducted for each sample under a reactant stream containing CO, H-2, CO2 and H2O. Negative effects of the dopants on the CO PROX activity were experimentally observed. Several characteristics on the catalysts induced by the insertion of dopants caused the effects. The presence of the non-reducible dopants on the surface hindered the efficiency of the redox equilibrium between copper and cerium, which is the essential process for CO PROX. Surface oxygen vacancies, generated by the introduction of foreign dopants, were not beneficial to the CO PROX activity. Copper species on the catalysts might be penetrated through these vacancies and protected as non-reactive reduced form in these vacancies. [GRAPHICS]