The hydrogenation of benzene adsorbed on Y zeolites containing various low platinum contents has been followed by u.v. and i.r. spectroscopies. Hydrogenation involves benzene which is irreversibly adsorbed at room temperature on PtNaY, PtHY or PtCaY samples. The reaction was performed in a static system with a constant and known hydrogen pressure. The concentration of benzene adsorbed on the zeolite decreases linearly as a function of the time of contact under hydrogen. Experiments using CO as a test molecule suggest that the platinum particles are covered by C₆H₆ during the hydrogenation process. By varying the hydrogen pressure, the rate of the reaction was found to be proportional to H₂ pressure. It was concluded that the hydrogenation reaction takes place on the metal: hydrogen is dissociated on the platinum particles and C₆H₆ weakly adsorbed on the support can migrate towards the metallic phase to be hydrogenated. Determination of turnover number, N, shows an increase of N with platinum content suggesting either the presence of thermal effects or changes of catalytic activity with the metal particle size.