The hydroisomerization of ethylbenzene (EB) has been carried out on a series of bifunctional Pt/Al2O3-zeolite catalysts, in which both the structure (mordenite, beta, Y, ZSM-5, MCM-22) and the chemical composition (for the mordenite and beta samples) of the zeolitic component were varied. Postsynthesis methods such as ion-exchange, steaming and chemical treatments were used to modify the composition of the samples (framework and extraframework Al content and nature of the compensation cation). Both parameters, zeolite structure and chemical composition, were seen to affect the activity and selectivity of the resultant catalysts. Large pore zeolites, particularly mordenite and beta, presented the highest selectivity to the desired isomerization products, i.e., xylenes. In the case of mordenites the textural properties (mainly the mesoporosity), played a key role in determining the activity and selectivity of the catalysts. Both, zeolite acidity and mesoporosity determined the amount of Bronsted sites accessible to the reactant molecules. It was seen that a reduced amount of accessible Bronsted acid sites in the zeolite favored the isomerization of EB with respect to secondary reactions like cracking of the naphthenes and dealkylation. Selectivities to xylenes above 40% at 60% EB conversion were obtained with most of the mordenite-based catalysts. Also, the calcium-exchanged mordenite samples gave larger xylene yields than the corresponding protonic samples. In the case of the Pt/Al2O3-beta samples, it was seen that formation of xylenes by isomerization of EB is favored in catalysts presenting a reduced density of Bronsted acid sites and a high mesoporosity. Thus, those catalysts prepared from the beta samples obtained by steaming and steaming + acid treatment of the original acidic zeolite were the most selective to xylenes, giving a selectivity close to 40% at ca 60% EB conversion.