A new type of "ship-in-the-bottle" catalyst has been obtained by encapsulation of heteropolyacid H3PW12O40 in the supercage of faujasite. As shown by XRD and IR, the structure of zeolite Y after encapsulation is retained. P-31 MAS NMR reveals that the signal at -14.4 ppm, assigned to the Keggin unit, is in the case of the encapsulated catalyst much broader than the line observed for a sample containing the heteropolyacid adsorbed on the zeolite. The broadening is due to interaction between phosphorus and aluminium from zeolite. Highly active catalysts are formed by encapsulation, as exemplified here by strongly increased activity and modified selectivity in isomerization and disproportionation of m-xylene.