The interaction of CO with silica supported molybdenum atoms has been studied by means of density functional calculations and cluster models. Experimentally two bands in the IR spectra of adsorbed CO have been observed at 2170 and 1990 cm(-1) with vibrational shifts of +27 and -153 cm(-1), respectively, with respect to the gas-phase molecule, the peak at +27cm(-1) has been related to the presence of neutral Mo atoms anchored to two oxygen atoms of the SiO2 substrate. Possible reactive sites at the Mo/SiO2 interface have been explored as candidates for CO adsorption. Mo atoms in various formal oxidation states, from +II to +VI, have been considered. Both molecular and cluster models of the Mo/SiO2 interface have been employed. The analysis shows that a neutral Mo(II) atom, proposed to be responsible for the blue-shift of v(CO), is not likely to be the origin of the IR band at 2170 cm(-1). Only Mo atoms in high oxidation states or Mo cations carrying a real positive charge can account for the positive shifts in the CO frequency.