Catalytic activity and surface acidity during the oxydehydrogenation of propane over a VPO/TiO2 catalyst were determined by a dynamic in situ FT-IR spectroscopy technique at 350 degrees C. Pyridine was used as a probe molecule for the acidity measurements. The obtained results show that propene formation is linked to Bronsted acid sites and that water increases the number of these sites which, in turn, increases propene selectivity.