This work describes a novel citrate procedure to prepare highly active and selective copper oxide-ceria catalysts for the preferential oxidation of CO in the presence of hydrogen. At the conditions studied, the prepared catalysts are very active and selective at temperatures around 160 degrees C, even though suffer from slow deactivation during reaction, which is ascribed to the progressive loss of active sites by copper oxide reduction. In spite of this, the catalytic activity of these catalysts is comparable to or higher than that of the most active catalysts reported in literature. Cobalt and manganese oxide-based catalysts prepared by the same procedure present a much lower catalytic activity than that of the copper oxide-based catalyst. The catalysts are formed by small arrangements of ceria crystals (2-40 crystals per particle) of nanometric size (4-10 nm), partially covered by XRD amorphous clusters of copper oxide. The presence of copper oxide reduces the normal growing of the ceria crystals during calcination and induces the agglomeration of such crystals. Raman spectroscopy was used to study the catalytic activity-related structural characteristics of the catalysts. Only ceria-related peaks were detected in the Raman spectra. The parameters of the main peak (at similar to 450-464 cm(-1)) were found to depend on the crystal size and on the extent of copper oxide covering of the ceria surface. On the other hand, the relation existing between the area of the second peak (at similar to 585-598 cm(-1)) and the amount of oxygen vacancies permitted to analyse their effect on the activity of the catalysts prepared at different conditions. (c) 2004 Elsevier B.V. All rights reserved.