NiMo sulfide combined with an acidic HY zeolite constitutes a bifunctional catalyst which may be used in alkane hydroconversion. In this work the distribution of Ni and Mo ions loaded in two HY zeolites (HY-3 and HY-17, with Si/Al = 3 and 17, respectively) is examined by transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, and N-2 adsorption. With zeolite HY-3, all the Ni but only half the Mo is found inside the grains, mainly in the cavities; MoO3 microcrystals are observed outside the grains. Sulfidation transforms the outer part of the MoO3 crystals into MoS2, but small MoS2-like particles (7 to 15 nn long, 3 slabs thick) are also detected inside the zeolite, mostly in the secondary pores. No significant migration of Mo occurs, but only 50% of the Ni is retained in the sulfided zeolite. Zeolite HY-17 is able to accommodate all the Ni and Mo ions, mainly in the large mesoporous volume. Sulfidation evicts 50% of Ni and ca. 30% of Mo from the zeolite particles. Again, MoS2-like entities (30 to 40 nm large, 6 to 10 slabs thick) are observed inside the grains. The dispersion of the NiMo sulfide in the HY-17 zeolite appears to be higher than in the HY-3 support. The NiMoY catalysts were evaluated for the simultaneous hydrogenation of benzene and hydroconversion of n-heptane. Owing to the good dispersion and accessibility of the NiMo sulfide, the zeolite catalysts are as active as NiMo/Al2O3 for hydrogenation. This hydrogenation activity, however, is unable to impart an adequate selectivity in hydroconversion of heptane.