Two catalysts were prepared(ca. 1 wt% Ir), one with metal particles located on the external surface of zeolite NaY and the other with metal particles located inside the zeolite supercages. The two catalysts exhibited very different activities and selectivities for the hydrogenolysis of n-butane, with the Ir in NaY catalyst much less active and less selective (53% ethane) than the Ir on NaY catalyst (83% ethane). The differences in catalytic activity and selectivity between Ir on NaY and Ir in NaY are attributed to differences in the size of the active metal particle, an ensemble effect, and not to the location (encagement) of the particle.