A temperature-programmed reduction (TPR) study of three series of biphasic hydrotreating (HDT) catalysts - PtS/gamma Al2O3/(PtS/gamma-Al2O3 + MoS2); Rh2S3/gamma-AlOOH/(Rh2S3/gamma-AlOOH + WS2) and PdS/gamma-Al2O3/(PdS/gamma-Al2O3 + WS2) - is presented. In all cases, the quantity of H2S produced is larger than that which would correspond to the arithmetic addition of the quantities produced by each of the sulfide phases alone. This fact suggests that the noble metal sulfide dissociates the hydrogen molecule to spillover hydrogen (H-SO), the latter producing a deeper reduction of molybdenum sulfide or tungsten sulfide. The similitude of catalytic effect in hydrogenation (HYD) and hydrodesulfurization (HDS) observed with the same mechanical mixture strongly suggests that H-SO also plays a role in the synergy between MoS2 or WS2 and the noble metal sulfides.