The H-D isotopic exchange between H-2 and D-2 and between H-2 (D-2) and D2S (H2S) was studied at 80 degrees C in a recycling reactor under a pressure of 2 bar in the presence of a sulfided NiMo/Al2O3 catalyst and by means of gas chromatography. From the H-2-D-2 exchange an amount of three exchangeable surface hydrogen atoms retained by the catalyst per Mo atom was estimated after sulfiding the catalyst by 10% H2S in H-2 at 400 degrees C and sweeping it by helium at 80 degrees C. H-D isotope exchange occurred between H-2(D-2) and D2S(H2S). As expected under the reaction conditions, H2S(D2S) did not produce H-2(D-2) The isotope exchange between H-2 and D-2 was about six times faster than the exchange between H-2(D-2) and D2S(H2S). However, it was shown that H2S competed with H-2 for adsorption : H2S inhibited the exchange between H-2 and D-2. It is proposed that H-2(D-2) and H2S(D2S) dissociate heterolytically on the same sites, involving both a sulfur vacancy and a sulfur atom. Furthermore, it is suggested that the isotope exchange could occur by dissociation of both reactants on one single center provided that this center possesses at least two coordinative unsaturations.