The highly acidic H-MFI zeolite effectively catalyzes the decomposition reaction of t-butyl hydroperoxide (TBHP) which most likely proceeds through an electrophilic attack of the zeolite protons on either oxygen of the peroxide. Replacement of the zeolitic protons by cationic Ga leads to a decrease of the reaction rate due to the removal of proton sites which also suggests that gallium cations are virtually inactive for TBHP decomposition. In contrast, copper ion-exchanged MFI is an active TBHP decomposition catalyst and, more importantly, radicals are involved as intermediates in the decomposition mechanism. The existence of radicals in the liquid phase was unambiguously evidenced utilizing N-phenyl-beta-naphthylamine as a radical inhibitor in the batch reaction. The involvement of a homolytic pathway in the decomposition reaction is explained with oxidation-reduction cycles of the zeolite copper cations.