Phase concentrations, crystalline structures and surface or bulk reduction properties of zirconia and ceria-doped zircoina nanophases prepared via a sol-get method were studied by X-ray diffraction (XRD) and temperature-programmed reduction (TPR) techniques. Rietveld refinement showed that both the phase composition and crystallite size of the solids depended on ceria content which related to the degree of surface and bulk reducibility. The pure zirconia and the solid doped with 1 wt% ceria consisted of tetragonal and monoclinic phases, while, the solids doped with 25 and 50 wt% ceria contained only one phase with cubic structure. Cerium incorporation into zirconia led to a crystalline structure distortion and reducibility enhancement of the resultant solids. The Rh loaded zirconia and ceria-zirconia solids would dissociate hydrogen and spillover it onto the support, lowering the temperature for both surface and bulk reduction of the support. A structural dependence of the catalytic activity for CO oxidation upon the catalysts support was observed. Catalytic activity of the Rh loaded zirconia-ceria solid solution with cubic phase is quite higher than for the catalysts with the support containing tetragonal and monoclinic phases of zirconia. The latter exhibited an inducing period in the reaction temperatures below 180 degrees C on the catalytic activity profile that might be a result of the relatively low reducibility of these support. (c) 2005 Elsevier B.V.. All rights reserved.