12-Tungstophosphoric acid (PW 1 2 ) has been supported on dealuminated zeolite Y. The catalysts were studied by XRD, DTA/DTG, IR, argon adsorption and MAS NMR spectroscopy. The structure of dealuminated faujasite is retained upon contact with a very strong PW 1 2 acid, which becomes thermally stabilized. Two types of Keggin anions exist at the surface: those strongly interacting with the OH groups of the support, prevailing at low coverages, and those weakly interacting with the zeolite, prevailing at high coverages. Transformations of m-xylene proceed along three pathways: isomerization, disproportionation and dealkylation. Pure zeolite shows high selectivity to isomerization. Deposition of PW 1 2 acid dramatically increases the selectivity of disproportionation to toluene and trimethylbenzenes, which rises with the growing acid loading.