H-D isotopic exchange between H-2 and D-2 was carried out at 80 degrees C in a recycling reactor under a pressure of 2 bar over unsupported MoS2 as well as on alumina supported Mo or NiMo sulfide catalysts. H-2-D-2 experiments carried out over a commercial NiMo/Al2O3 catalyst, to which were mixed different amounts of the alumina support, and over a series of Mo/Al2O3 catalysts containing different amounts of Mo (4.4 to 14.8 wt%) showed that part of the exchangeable hydrogen detected by isotopic dilution came from the added alumina or from the support. This amount of hydrogen on the alumina was estimated for each series, to be respectively 22.1 x 10(-4) and 17.8 x 10(-4) mol H . g(-1). Since under the same conditions H-D exchange between H-2 and D-2 did not occur at an appreciable rate on the alumina alone, it is concluded that hydrogen atoms from structural hydroxyl groups of the alumina or from hydroxyl groups resulting from the dissociation of H2S were able to migrate from grain to grain and to exchange with H-2 (D-2) dissociated on the sulfide active phase. The hydrogen atoms supposedly migrate as protons by jumping from oxygen to oxygen atoms on the alumina surface or are conveyed through the gas phase by H2S (the "shuttle molecule") which can adsorb dissociatively both on the alumina support and on the active phase. D-2 adsorption followed by FTIR under similar conditions confirmed that the presence of the sulfide phase was necessary, under the conditions of the experiments, to make the isotopic exchange between H-2 (D-2) in the gas phase and the hydroxyls of the alumina possible. OD bands were difficult to detect on sulfided alumina alone, even after 5 h of exposure to D-2, whereas a strong OD band appeared very rapidly on sulfided NiMo/Al2O3. These results are considered as being in favour of a heterolytic splitting of H-2 On sulfide catalysts.