Pyridine adsorption is used to study the acidic sites of silica-supported CuO-MoO 3 catalysts, with reference to CuO/SiO2 and MoO3/SiO2 catalysts. At low wt% of copper, both Lewis and Bronsted acid sites are present on the CuO-MoO3/SiO2 surface. A substantial decrease and then vanishing of the Bronsted acidity is obtained as a result of increasing the copper loading. An approach confining the prevalence of Lewis acidity over the Bronsted ones is described and discussed. A new band of Lewis acidity is observed at 1574 cm -1 and attributed to the presence of CuMoO4 species. This band is not observed in the individual oxide form of both CuO/SiO2 and MoO3/SiO2 catalysts. Carbon monoxide adsorption on CuO-MoO3/SiO 2 catalysts reflects a major dispersion if it is compared with CuO/SiO: and MoO3/SiO~ catalysts and also gives a picture about the expected structures of these supported oxide catalysts. Copper is found also to exert a major control on the mode of interaction of molybdenum species with the silica surface. Moreover, important correlations and shifts in the IR bands are evaluated and discussed.