The role played in the transformation of the xylene isomers by each of the pore systems of a MCM-22 (MWW) zeolite was determined by selectively deactivating the protonic sites of the supercages and poisoning those of the external hemicages with 2,4-dimethylquinoline. In the large supercages (7.1 Angstrom circle divide, 18.2 Angstrom height) with narrow apertures (4.0 Angstrom x 5.5 Angstrom), the three isomers undergo isomerization I and disproportionation D. the first reaction being largely predominant (D/I ratio of 0.3 from the three isomers). The desorption of trimethylbenzene D products being strongly limited, a large part of them undergo secondary transformations leading to less bulky products and to "coke". The isomer distribution is that expected from a product shape-selective process. Furthermore, within the narrow sinusoidal channels (4.1 Angstrom x 5.1 Angstrom), only isomerization of meta- to para-xylene can occur; the ortho-xylene molecules are unable to enter (or to desorb from) the channels, which was confirmed by adsorption followed by IR spectroscopy. Lastly, in the large hemicages, the main reaction is non-shape-selective isomerization. The distribution of protonic sites within the three pore systems was estimated from the results of o-xylene adsorption and 2.4-DMQ poisoning experiments. The comparison of this distribution with the respective catalytic role of the different pore systems shows that the external hemicage protonic sites are 2-3 times more active than those of the other pore systems. This observation can be related to the complete absence of diffusion limitations in xylene transformation within the external hemicages. (C) 2004 Elsevier B.V. All rights reserved.