The properties of alumina-supported Mo and NiMo sulfides toward a hydrogen atmosphere were investigated in order to elucidate if the synergy observed in hydrotreating reactions might be related to the activation of hydrogen. The determination of the amount of adsorbed hydrogen and of the H-2-D-2 exchange rate for various reduced states has evidenced that Ni does not promote the reducibility and the amount of adsorbed hydrogen of the MoS2 phase while the exchange rate is increased only by factor of 2-3. It is concluded that the synergy observed in hydrotreating reactions does not come from hydrogen activation limitation.