CuCl has been dispersed in the supercages of a Y-type zeolite by heating a mechanical salt/host mixture in vacuo. The occluded salt was subsequently reduced to copper metal in a hydrogen atmosphere. Virtually complete reduction of the salt is achieved at 460°C. Under the same conditions, extraframework copper(II) ions, exchanged into zeolite NaY, are only partly reduced. The copper forms nanoaggregates of narrow size distribution inside the zeolite pore system; the average particle diameter is 5 nm. Our data suggest that these nanoparticles consist of several interconnected copper assemblies of supercage (diameter 1.2 nm) size. A small fraction of the salt remains at the outer surface of the zeolite crystallites in the inclusion step, and there produces larger copper metal particles upon reduction with H2.