The acid-base properties of pure and fluorinated alumina were determined by temperature programmed desorption (TPD) of NH3 and by infrared spectroscopy (IR) of adsorbed pyridine. A correlation between the acidity of the samples and the samples' catalytic activity in the alkylation of benzene with propylene was found. However, there was not a direct correlation between total acidity and activity and poisoning the catalysts with increasing amounts of sodium ions indicated that only the Bronsted acid site appears to be involved in the catalytic process. From physicochemical characterizations, it is proposed that such Bronsted sites are associated with hydroxyfluorides and aluminotrifluorides formed during the fluorination of the alumina.