The acidic and catalytic properties of alumina as well as fluoride-and sodium-modified alumina were characterized by FT-TR spectroscopy of adsorbed pyridine and t-butylcyanide (TBC), and by cumene and thiophene decomposition. Experimental evidence demonstrated that Al2O3 contains a high number of Lewis acid sites. A higher number of these sites of higher strength as well as Bronsted acid sites were detected for Al2O3-F-. In the case of Al2O3-Na+, the concentration of Lewis acid sites was lower compared to Al2O3 and also the acid strength was weaker. The Al2O3-F- sample showed high activity in cumene dealkylation whereas Al2O3-Na+ was most active in cumene dehydrogenation. The most active catalyst for thiophene hydrodesulfurization was Al2O3-Na+ while Al2O3-F- was a little less active and lost its original activity rapidly. It was suggested that in the first step a thiophene molecule is chemisorbed vertically through a sulfur atom on Lewis acid sites localized on the alumina surface.