A VPO/TiO2 catalyst tested in the oxydehydrogenation reaction (ODH) of propane between 300 and 400 degrees C shows satisfactory performances (up to 80% of propene selectivity at 2% of propane conversion at 300 degrees C or 56% of propene selectivity at 9% of propane conversion at 400 degrees C). Addition of water or pyridine in the feed gas tends to decrease the propane conversion and enhances the propene selectivity. It is shown that water increases the number of Bronsted surface acid sites by dissociative adsorption which, in turn, enhances propene selectivity at the expense of the COx selectivity. These results are in good agreement with spectroscopic IR observations performed under catalytic conditions showing that the Lewis acid sites are linked to COx formation, whereas it seems that Bronsted sites would rather be linked to propene formation.