The acidic and catalytic properties of alumina as well as fluoride- and sodium-modified alumina were characterized by FT-IR spectroscopy of adsorbed pyridine and t-butylcyanide (TBC), and by cumene and thiophene decomposition. Experimental evidence demonstrated that Al 2 O 3 contains a high number of Lewis acid sites. A higher number of these sites of higher strength as well as Brönsted acid sites were detected for Al 2 O 3 F − . In the case of Al 2 O 3 Na + , the concentration of Lewis acid sites was lower compared to Al 2 O 3 and also the acid strength was weaker. The Al 2 O 3 F − sample showed high activity in cumene dealkylation whereas Al 2 O 3 Na + was most active in cumene dehydrogenation. The most active catalyst for thiophene hydrodesulfurization was Al 2 O 3 Na + while Al 2 O 3 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.