An infrared spectroscopic study was carried out on a MoO₃—Fe₂(MoO₄)₃ catalyst. From the bands observed, water was shown to be molecularly adsorbed at room temperature. After heating at 100°C, water is desorbed, and absorption bands typical of H^- bonded hydroxyls were observed. The intensity of these bands decreases by further heating; at 400°C no was band detected at all in the ranges 2500-4000 cm^-1 and 1500-1800 cm^-1. After adsorption of ammonia at room temperature a broad band at 1140 cm^-1 appears, which was attributed to the vibration of ammonia interacting with adsorbed water. This band disappears by simple evacuation. On catalyst heated at 100°C and 250°C, after ammonia adsorption we have observed bands typical of the ammonia adsorbed on Lewis acid sites. On the catalyst heated at 100°C evidence was also obtained for the presence of Bronsted acid sites. It was concluded that, at temperatures around 250°C, the surface acidity of the catalyst mostly arises from Lewis sites, which probably are anionic vacancies formed by dehydroxylation, as occurs in Al₂O₃.