We have compared the acidity and the catalytic properties of (i) zeolite H-Y, (ii) H-Y dealuminated by hydrothermal treatment (H-US-Y), and (iii) H-US-Y realuminated by the KOH treatment (H-Real-US-Y). Quantitative IR measurements of pyridine sorption show that the concentration of "3460 cm(-1) hydroxyl groups", the Bronsted acid sites active in many catalytic reactions, is slightly lower in sample H-Real-US-Y than in H-Y. This demonstrates that most of the 3460 cm(-1) hydroxyl groups, present in the parent material and subsequently destroyed by dealumination, are restored by realumination and the K+/H+ exchange. The turnover frequency of the realuminated zeolite in m-xylene reactions is higher than in the parent sample, as a result of the higher strength of the acid sites, caused by the higher population of the most strongly acidic Si3Si-OH-AlSi3 groupings. Dealumination and realumination also alter the selectivity in the transformation of m-xylene. Disproportionation of xylenes to toluene and trimethylbenzenes, which involves bulky transition step complexes, is hindered in the dealuminated zeolite, where the channels are partly blocked by the extra-framework Al species, but is restored in the realuminated material, in which the extra-framework species are re-inserted into the framework.