The effects of coking and surface modification of H-ZSM-5 zeolites on enhancement of para-diethylbenzene selectivity during disproportionation of ethylbenzene have been investigated. Surface modifications of the zeolites by silica chemical vapor deposition (Si-CVD) and by stepwise lepidine adsorption and/or Si-CVD treatments were examined. The acidity, sorption capacity, stability, and coke content of the samples were characterized by various techniques, such as adsorption, TGA, IR, Xe-129 NMR, and solid-state Al-27 and P-31 MAS NMR. The performance of the para-selective process was studied by the correlation between the observed para-selectivity and conversion. The working principle responsible for the para-selective feature was found due to the combined effects of diffusion limitations and inactivation of external active sites. At the extreme of low conversion, the feature depends mostly on the former effect, whereas the latter becomes progressively important with increasing conversion. At high conversion, while both effects are important, diffusion controlled limitations associated with steric hindrances near the pore mouths plays the predominant role, especially towards saturating surface SiO2 loading. (C) 2002 Elsevier Science B.V. All rights reserved.