V,05/A1203 catalysts of different vanadium contents were prepared by contacting powdered y-A&O, with aqueous solutions of different concentrations of NH4V03. Calcined catalysts with vanadium loadings below the monolayer coverage of A1203 possessed adsorbed V-oxide layer which resisted dissolution in an ammoniacal solution. On the other hand, those catalysts of higher vanadium loadings had adsorbed V-oxide layer as well as precipitated crystalline V205. The latter preferentially dissolved in NH,OH solution. Infrared studies of chemisorbed NH3 on V-Al-oxide catalysts as well as temperature programmed desorption (TPD) of NH3 from these materials are reported. These studies revealed the existence of two hinds of both Lewis and Bronsted acidic sites. Below the monolayer coverage, Lewis acidity originated from Al ions uncovered with vanadia, whereas at high loadings of vanadia, this acidity is attributed to the unsaturated vanadyl groups. Regarding the Bronsted acidity, the V-OH species of the adsorbed V-oxide and the V-OH species of precipitated V205 were shown to be responsible for this acidity. TPD results showed that the NH, was desorbed from Lewis acid sites by heating at = 440 K and from Bronsted acid sites of adsorbed V-oxide at = 640 K. At higher loadings of vanadium, the Lewis acidity was diminished due to coverage with precipitated VzOs, whereas Bronsted acidity of these precipitates was predominant. Ammonia was desorbed from the Bronsted acidic sites of V,O, particles by heating at = 530 K. The various hinds of surface acidic sites were quantified by analysing the desorbed NH3 from each kind of these sites. The present results assisted in confirming a proposed structure of the V-Al-oxide catalyst.