A series of Al-MCM-41 mesoporous materials with different Si/Al ratios are synthesized. The acidities of catalysts are measured by the temperature-programmed desorption of ammonia (NH3-TPD) and IR spectra of pyridine. Their catalytic activities for hydrogenation of benzene, toluene and o-xylene are investigated on a pulse reactor system. NH3-TPD and IR results show that only weak and medium acid sites could be observed on the catalysts, and the number of total acid sites decreases obviously with the increase of the Si/Al ratio whereas the medium acid sites are somewhat constant. The introduction of platinum onto Al-MCM-41 material decreases the total acid number by a small amount. The hydrogenation activities of the 1% Pt/HAl-MCM-41 catalysts are found to correspond well with the ratio of medium acid sites to total acid sites in the employed catalysts. It is proposed that, in addition to metal sites, the acid sites in the metal-acid interfacial regions are also the active sites for hydrogenation and the medium acid sites play important roles in the reaction.