Desilication accompanied with minimum loss of crystallinity effect of a high alumina ZSM-5 zeolite on the isomerization reaction of ethylbenzene/xylene mixtures has been considered. Desilication was assessed through XRF, XRD, FTIR, TEM, nitrogen adsorption/desorption, NH3-TPD, 29Si and 27Al MAS NMR analytical techniques. Desilication was accompanied with the creation of super acid sites. There exists a limit (Si/Al molar ratio of 9.67) for keeping high crystallinity and obtaining improved catalytic performance. Desilication promotes ethylbenzene conversion by disproportionation and trans-alkylation reactions while the same reactions are limited for the xylene isomers. The p-xylene approach to equilibrium improves by more than 7% at 400°C and a WHSV of 2h−1 for the optimum sample with respect to the parent zeolite. At the same conditions, the optimum sample exhibits the maximum ethylbenzene conversion of 89%, i.e. more than 40% w.r.t. of the parent zeolite. However, the xylene yield decreases only 3%.