m-Xylene transformation was carried out at 623 K over a series of commercial HFAU zeolites with framework Si/Al ratios between 4 and 100. All the samples deactivate rapidly owing to the formation of coke. The lower the Si/Al ratio the faster the deactivation, which can be explained by the increase in acid strength when the Si/Al ratio decreases and by the faster deactivation of the strong acid sites shown by the effect of cake on the intensity of the various OH IR bands. Deactivation causes a significant increase in the disproportionation/isomerization rate ratio (D/I) and a significant decrease in the para/ortho selectivity (p/o). These selectivity changes can be related to a greater selectivity of the strong acid sites in favor of the classical monomolecular mode of isomerization at the expense of disproportionation and of the bimolecular mode of isomerization (through m-xylene disproportionation followed by m-xylene-trimethylbenzenes transalkylation). This was confirmed by the fact that pyridine poisoning and deactivation by coke deposition have an identical effect on D/I and p/o selectivities. The rapid transformation into coke of the diphenylmethane intermediates of disproportionation and of bimolecular isomerization on the strong acid sites was proposed in order to explain the low selectivity of these sites for these reactions.