The disproportionation reaction of xylenes, catalyzed by acidic zeolites, is investigated by means of periodic ab initio calculations. Two relevant 10- and 12-membered ring (MR) frameworks are chosen: theta-1 (TON) and mordenite (MOR). As a first step, two bimolecular mechanisms proposed for this reaction are investigated: the first one is assumed to occur through a one-step methyl shift and involves a highly unstable methyl cation as a transition state. The second and energetically favored mechanism implies the formation of a benzylic-like carbocation, subsequently converted into a trimethyldiphenylmethane, also called DPM intermediate. The energetics of this reaction path are mainly governed by the different sterical constraints induced by the 10-MR or 12-MR zeolites on the DPM intermediates. Therefore, the relative stability of the DPM inside the pores appears as the key parameter for understanding the selectivity observed experimentally. (C) 2003 Elsevier Inc.