The modes of coking and deactivation of a USHY zeolite during m-xylene transformation at 523 and 723 K were established. At both reactions temperatures, coke molecules result mainly from transformation of the diarylmethane intermediates of transmethylation reactions, olefinic products formed only at 723 K also participating in coke formation at this temperature. From the effect of the number of coke molecules on the accessible micropore volume, on the hydroxyl bands, and on the number of protonic sites able to retain adsorbed pyridine, it can be concluded that at both temperatures, deactivation is initially due to poisoning of the protonic sites, pore blockage by coke deposits appearing at longer time-on-stream. The effect of this latter mode of deactivation is more significant at 723 than at 523 K. No interaction of Lewis acid sites with coke molecules is observed, which suggests that these sites play no role in m-xylene transformation and coke formation.