Ultrastable zeolites (USY) have been treated with sulphuric and/or phosphoric acids, aiming at the determination of the interaction of both acids with either framework (FAL) or extraframework (EFAL) aluminas, as well as the effect of such modifications on catalytic properties in ethylbenzene disproportionation. Zeolites have been characterized by means of several physicochemical techniques (XRD, XRF; FTIR, Si-29, Al-27 and P-31-MAS/NMR, nitrogen adsorption and AA). Results revealed that the treatment with H2SO4 removes EFAL located in supercavities without attacking the zeolitic framework. Treatment with H3PO4 incorporates P in two quite distinct ways : as a monomeric phosphate associated to framework aluminium atoms (for low EFAL concentrations) and as a polymeric phosphate originated from the reaction of EFAL with H2PO4 (for high EFAL concentrations). Regarding catalytic properties, initial activities are higher for H2SO4 leached samples when compared to those which have just been calcined. Nevertheless,initial activities of samples treated with H3PO4 depend strongly on the P2O5 content incorporated into the zeolite. With respect to isomers (ortho-, meta- and para-diethylbenzenes) generally formed in this disproportionation reaction, no considerable para-selectivity has been observed.