H2–D2 equilibration was used to study the synergetic effect between Co and Mo in carbon-supported CoMo sulfide catalysts. The reaction was carried out in a recirculation apparatus with continuous on-line analysis of all gas-phase components (H2, D2, HD, H2S, D2S, and HDS) by a mass spectrometer. The low reaction temperature (423 K) allowed for the first time a comparison to be made of the catalytic activity between Co sulfide particles in low-temperature sulfided (373 K) carbon-supported Co and those in standard sulfided (673 K) carbon-supported CoMo, which have similar 57Co MES parameters. On the basis of the much higher activity of the bimetallic catalyst than of the monometallic catalysts (Co and Mo sulfide) it is concluded that the observed synergy is related neither to the effect of the high dispersion of the Co sulfide nor seemingly to spillover effects as put forward by the remote control model. The results indicate that the sulfur atoms that bridge between Co2+ and Mo4+ play a crucial role during the catalytic cycle. During the equilibration reaction also extensive exchange takes place between hydrogen (deuterium) atoms adsorbed on the sulfided phase and hydrogen atoms present at the carbon support surface. This spillover process is accelerated by admixture of H2S, which most probably leads to an increase of terminal SH groups by dissociative H2S adsorption. These SH groups are thought to form hydrogen bridges with the carbon support functional groups. Both H2 and H2S compete for adsorption on the same coordinatively unsaturated sites. While heterolytic dissociation of H2 is considered to lead to a hydride and a bridging SH group between Co and Mo, heterolytic dissociation of H2S would lead to the formation of two SH groups, one bridging SH group and one terminal SH group active in forming hydrogen bridges with the support surface functional groups.