In order to better understand the role of the acidity of the support on the hydrogenation properties of a molybdenum sulfide phase, a beta-zeolite with a nominal Si/Al = 13.8 was partially (Si/Al = 15.0 and 18.7) and fully dealuminated (Si/Al greater than or equal to 800, no Bronsted acidity). To achieve a reasonable Mo loading with a high Mo dispersion, the Mo/beta catalysts were prepared by cation exchange with [Mo3S4(H2O)(9)](4+) in the presence of NH3, followed by a thermal treatment under Ar at 623 K. This preparation method allowed us to introduce up to 4.7 wt% of Mo into the acidic zeolites. For the fully dealuminated zeolite that contains no exchange sites, Mo was introduced by impregnation with an aqueous solution of ammonium heptamolybdate. After sulfidation of all the samples in a stream of H-2/H2S, MoS2 slabs were observed by TEM. The electronic properties of the molybdenum sulfide phase were examined by means of IR spectroscopy of CO adsorption at 100 K. The nu(CO) wavenumber regularly increases with the acidity of the zeolite support from 2122 cm(-1) for the nonacidic sample to 2158 cm(-1) for the most acidic one. The hydrogenation activities of these catalysts determined in tetralin hydrogenation, and carried out in the presence of H2S, also varied within a wide range, the most acidic catalyst being circa 40 times more active than the nonacidic one. This enhancement of activity in hydrogenation is related to the electron-deficient character of the sulfide particles in acidic zeolites. (C) 2003 Elsevier Inc. All rights reserved.