NaY-supported molybdenum sulfide catalysts with increasing Mo loading have been prepared by impregnation with ammonium heptamolybdate and characterized by means of thiophene hydrodesulfurization (HDS), overall sulfur analysis, temperature programmed sulfidation, N-2 adsorption, Xe adsorption, Xe-129-NMR, and high resolution electron microscopy, After impregnation nearly all Mo is located on the exterior of the zeolite particles, During calcination in air part of the Mo is redistributed, resulting in a higher amount of Mo being located in the zeolite supercages. For the sulfided catalysts an amount of Mo comparable to that after calcination is present in the supercages. At the highest Mo loadings the catalysts are incompletely sulfided, due to the presence of large "bulk-like" molybdenum oxide particles on the outside of the zeolite particles. The amount of Mo present in the supercages, as well as the thiophene HDS activity of the sulfided catalysts, increases with increasing Mo loading. The small Mo sulfide particles in the supercages contribute strongly to the thiophene HDS activity.