From kinetic studies of the catalytic hydrodemethylation of toluene on clinoptilolite exchanged by K +, Mg 2+, and La 3+ ions, respectively, the mechanism of the reaction could be deduced. The rate-determining step of the transformation is a surface reaction in which hydrogen from the gas phase reacts with chemisorbed toluene in a presumably four-center reaction resulting in gaseous methane and adsorbed benzene. The kinetic parameters of the reaction i.e., the activation energies as well as the heats of adsorption of benzene or toluene are identical for all three catalysts, and suggest thus energetic homogeneity of the active sites. Possible active sites above 600 °C are probably the strongly electrophilic Lewis centers, or ensembles of Lewis centers coupled with neighboring SiO 3 + lattice groups. The presumption of such active formations is supported by the decrease of hydroxyl coverage and the high radical stabilizing power of the catalysts as proved by ESR measurements, and further by recognition of a linear relationship between the activity of the catalysts and their exchanged cation content. This correlation shows that bivalent or trivalent cations give rise to identical formations proportional in number to their equivalent amounts, the active sites formed being independent of the other properties of the respective cations.