Transalkylation of toluene with cumene was studied over a series of Y zeolites with framework SUM ratio of 2.5, 5.6, 12.3 and 21.2 at 453 and 493 K. Samples were prepared by progressive dealumination of the parent NH4Y(Si/Al = 15) zeolite in solid-state with crystalline (NH4)(2)[SiF6]. It was proved by IR spectroscopy that the replacement of framework aluminum by silicon resulted in an increase in the strength and the thermal stability of the acid sites of Broensted-type. These samples do not contain extra-framework aluminum species. The parent Y zeolite possessing the highest amount of weak Broensted acid sites showed low activity in the transalkylation reaction, and, in addition a considerable amount of reactants and products was found to adsorb on this sample at 453 K. The highest cumene conversion was achieved on the sample with SUM ratio of 5.6. The balance between the number and strength of the acid centers that assures an optimal activity appears to be disadvantaueous in respect to the selectivity. It was demonstrated that the reaction system is very sensitive towards these characteristics influencing not only the activity but also the catalysts' selectivity and stability. (C) 2003 Elsevier B.V. All rights reserved.