The acid-base properties of V2O5/gamma-Al2O3 catalysts were characterized by ammonia, pyridine and sulphur dioxide adsorptions using microcalorimetry and diffuse reflectance Fourier transform Ir spectroscopy (DRIFT). For vanadium content less than 10 wt.-% V2O5 (4.9 mumol(V2O5) m-2), the vanadium cations were found to be well spread as vanadate compounds over alumina. Such vanadate species developed Bronsted and Lewis-type acidity as shown by a DRIFT spectroscopy study of pyridine adsorption, but did not exhibit a basic character. Sulphur dioxide adsorption allowed to differentiate a vanadate layer from free alumina. Ammonia and sulphur dioxide adsorptions also showed that at low vanadium coverage, a large part of the vanadate layer was bound to acid-base pairs of alumina. Vanadium pentoxide crystallites were detected at less than complete monolayer coverage with vanadate compounds, but did not contribute to the development of the acidic surface character. At low vanadium coverage, (<3 wt.-% V2O5, 1.2 mumol(V2O5) m-2), the acidic character of the V2O5/gamma-Al2O3 catalysts is ascribed to vanadium-free alumina, whereas it is largely attributed to vanadate compounds at higher vanadium coverage.