Catalytic exchange of deuterium with benzene was used as a test reaction to investigate both the number and chemical nature of active sites on alumina. Previous work has involved deuterium exchange with paraffins and olefins on alumina. The results of this work are similar to those reported for the earlier studies. In comparison with hydrogen exchange with perdeuteriobenzene, a primary kinetic isotope effect of 1.8 was found at 20°. However, redistribution of "hydrogen" atoms among the benzene isotopes was much faster than either of the exchange reactions. The addition of a small amount of benzene reduced the rate of the usually fast hydrogen-deuterium exchange by 98%. The strong, dissociative adsorption of benzene to the exclusion of hydrogen is a possible explanation for these results. After catalyst pretreatment at 530°, "titration1" using carbon dioxide as a poison of catalytic activity indicated an active site density of about 15 x 10 to the 12th/cm2 for both the exchange and redistribution reaction. This number is close to that necessary to poison hydrogen-deuterium exchange, which indicates that all three reactions occur on the same or on closely related sites. Because C1402 poisoning demonstrated that other sites besides those active for exchange also adsorb carbon dioxide, the above active site density is only an upper bound. Poisoning with other materials had only a slight effect, which again may indicate that benzene was so strongly adsorbed that it excluded other molecules from the catalyst surface. The similarity of the exchange reaction over a deuterated catalyst ruled out the possibility of exchange with a majority of catalyst hydroxyl groups.