Deposition of carbonaceous residues and tetraethoxysilane has been used to passivate the external surface of nano-sized HZSM-5. Using a real C-8 aromatic feedstock, both modification methods let to a reduction of xylene loss during xylene isomerization, i.e., a decrease in undesired disproportionation reactions yielding toluene and trimethylbenzenes. The higher performance obtained for the pre-coked sample can be explained by a more efficient deactivation of strong external acid sites. The effect of surface modification on reaction mechanism was studied by using C-14-labeled toluene and ethylbenzene. At low ethylbenzene conversion, transalkylation reactions forming ethyl methylbenzene have been observed, whereas at higher conversion the additional build-up of aromatics from the ethyl group (splitted off during dealkylation) can be revealed by Radio-HPLC data. Experiments with C-14-labeled toluene confirm a supplementary bimolecular pathway of xylene isomerization via transmethylation reactions, which are assumed to take place predominantly on the external surface of medium-pore zeolite ZSM-5. (c) 2005 Elsevier B.V. All rights reserved.