While 1,2,4-trimethylbenzene (TMB) disproportionation is one of the potential technologies for p-xylene production, its reaction intermediates have been neither experimentally observed nor identified yet. Here we present GC-MS evidence that not only pentamethylated diphenylmethane (5mDPM) derivatives but also hexamethylated diphenylmethane (6mDPM) ones are serving as key reaction intermediates of 1,2, 4-TMB disproportionation over large-pore zeolites. We also propose a new bimolecular diphenylmethane-mediated reaction pathway for the formation of tetramethylbenzenes (TeMBs) and xylenes over large-pore zeolites based on the GC-MS results obtained. Comparison with the GC-MS results from LaNa-Y after transalkylation reactions of three different TeMB isomers with three xylene and three TMB isomers, respectively, allows us to rationally identify most of the seven 5mDPM and three 6mDPM derivatives at their isomer level. A combination of GC-MS and DFT calculation results demonstrates that the bimolecular 1,2,4-TMB disproportionation over LaNa-mordenite, a practically one-dimensional 12-ring zeolite with regard to this reaction, is a new example of transition state shape selectivity in zeolite catalysis. (C) 2014 Elsevier Inc. All rights reserved.