The influence of the Na+/H+ ratio on the benzene hydrogenation activity and the sulfur tolerance of Pt supported on mordenite (MOR) was studied by in situ XANES. An increase of the Pt white line was observed with increasing concentrations of thiophene up to 400 ppm, which indicates that an equilibrium exists between the thiophene concentration in the gas phase and the sulfur compounds present on the metal surface. Above a certain concentration of thiophene the Pt white line was constant, indicating a saturation of the metal surface with thiophene. The effect of sulfur poisoning on the metal surface was more effective for Pt supported on H-MOR compared to the more basic Pt/Na-MOR catalysts, which is attributed to the more pronounced decomposition of thiophene and the formation of H2S on Bronsted acid sites. During benzene hydrogenation, a decrease of the Pt white line resulting from the adsorption of benzene and other reaction intermediates on the Pt surface was observed. In the presence of sulfur, the increase of the Pt white line intensity during benzene hydrogenation suggests a higher sulfur coverage of the metal for Pt/H-MOR compared to Pt/NaH-MOR. An optimum concentration of acid sites was found with respect to the benzene hydrogenation rate and sulfur tolerance of the catalysts, which is attributed to the presence of two reaction pathways for benzene hydrogenation, i.e., one which is catalyzed by Pt and another in which acid sites close to Pt particles are the catalitycally active sites.