The present work provides solid-state C-13 NMR spectroscopic evidence that the zeolites acidity and the pore size strongly affect the catalytic behavior of ethylbenzene disproportionation and coke formation. The medium-pore zeolite H-ZSM-5 (ca. 0.56 nm) and the large-pore zeolite H-Y (ca. 0.74 nm) used in this study have exclusively Bronsted acid sites, but with different acid strength. Due to the transition state shape selectivity of ethylbenzene disproportionation, ethylbenzene transalkylation on H-Y takes place at low reaction temperature without side-reactions. On H-ZSM-5, dealkylation/realkylation was observed and generation of alkylcarbenium ions resulted in secondary reactions. These alkylcarbenium ions initiate coke formation on zeolite H-ZSM-5 via oligomerization, six-ring closure, and aromatization of alicyclic hydrocarbons. Ethylbenzene disproportionation on large-pore zeolites Y is an attractive reaction due to its low reaction temperature, high selectivity without side-reactions, and low coke formation. Medium-pore zeolite H-ZSM-5 showed higher reactivity for dealkylation of ethylbenzene and protolytic cracking of light alkanes due to its narrow channels and stronger Bronsted acid sites. (C) 2009 Elsevier Inc. All rights reserved.