The effect of the structure of large pore zeolites of Mordenite, beta, L and Y types on the activity, selectivity to xylenes and time-on-stream stability was investigated in trimethyl benzene disproportionation. Significant differences in conversion and time-on-stream stability were observed among individual zeolites. Zeolite beta and zeolite Y were found to exhibit the highest stability in conversion of trimethyl benzenes, while zeolite L and Mordenite were deactivated much faster. The optimum reaction conditions to achieve a high and stable conversion with sufficient selectivity to xylenes were at the reaction temperature of 400 degreesC with WHSV = 5-20 h(-1). Diffusion coefficients of trimethyl and tetramethyl benzenes over zeolites Y, beta and Mordenite at 25 degreesC were determined. Due to the significant differences in diffusion of individual trimethyl and tetramethyl benzenes, it is expected that diffusion of these molecules substantially influence the rate of this reaction. The coverage of acid sites by individual trimethyl benzene and tetramethyl benzene molecules differed significantly (beta > zeolite Y > Mordenite), which indicates large differences in the accessibility of acid sites located in the channel system of these zeolites. (C) 2004 Elsevier B.V. All rights reserved.