Adsorption of pyridine on MoO3/TiO2 doped with alkali metal cations (Li, Na, K, and Rb) has been studied by FT-IR spectroscopy in order to assess the effect of the nature and concentration of alkali cations on the surface acidity. It has been found that incorporation of the alkali cations leads to a decrease in the surface acidity and of the ratio between the concentration of Bronsted and Lewis surface acid sites. These decreases are due to (i) formation of weak Lewis acid sites (alkali cations) with lower electron affinity than Mo6+ and Ti4+ and (ii) cancellation of Bronsted sites (Mo-OH) because of formation of alkali molybdates. These effects are more pronounced as the alkali content increases and/or the polarizing character of the alkali cation is increased.