The e.p.r. spectrum of benzene on hydrogen mordenite (Zeolon H) has been studied as a function of coverage, degassing temperature, and hydrogen and oxygen treatment. The initial spectrum observed was that due to the benzene cation radical dimer. The maximum number of spins was formed when the solid was completely saturated with benzene. Large yields were obtained only when the zeolitic water and water of hydration of the protons were removed. The largest yields were observed where significant dehydroxylation had occurred. Chemisorbed oxygen was essential, and there was a 1:1.2 ratio between the maximum number of spins observed in the presence of excess oxygen and the amount of oxygen chemisorbed. The largest concentration of radical cations noted (2 × 1018/g) was much smaller than the number of acid sites known to be present (1021/g). In fact, only one unit cell of the hydrogen mordenite in 100 had a site capable of oxidizing benzene to the cation. Hence these sites must be in special positions in the solid.