CO adsorption at low temperature has been used to probe Lewis acid sites created upon dehydroxylation of gamma-Al2O3 and reduction of Mo/Al2O3 catalysts, using Fourier Transform Infrared spectroscopy (FTIR). Carbon-monoxide adsorption on gamma-Al2O3 and Mo/Al2O3 catalysts dehydroxylated and reduced at different temperatures was studied at 78 K by IR spectroscopy. However, our results indicate that there is an approximately linear correlation between the increase either of dehydroxylation or the extent of reduction of the catalysts and the increasing absorbance of CO due to CO adsorption on Lewis acid sites created upon dehydroxylation of gamma-Al2O3 and reduction of Mo/Al2O3.