CeO2-ZrO2 mixed oxide catalysts were prepared via a sol-PI technique and tested for carbon monoxide (CO) oxidation. Highly uniform nano-size solid solution particles of ceria-zirconia were attained under the conditions of this study (ca 100 degreesC). The stabilization of the surface areas of the catalysts can be achieved by the addition of zirconium. The CO oxidation activity of the mixed oxides was found to be dependent on Ce/Zr ratio, which relates to the degree of reducibility. The catalytic activity for CO oxidation decreases with a decrease in Ce/Zr ratio. This might be due to the difference in phase compositions of the mixed oxides. It can be postulated that the cubic phase, fluorite structure, which is mainly found in Ce1-xZrxO2 (where x < 0.5) could be reduced easily than the tetragonal phase found in Ce1-xZrxO2 (where x > 0.5). Among the mixed oxide catalysts, Ce0.75Zr0.25O2 was reported to exhibit the highest activity for CO oxidation.