The role of the promoter metals palladium and ruthenium in the mechanism of the hydrocarbonylation (reductive carbonylation) of methanol over cobalt/active carbon catalysts has been investigated. It has been found that the reduction steps on active Co sites are strongly affected by activated hydrogen transferred from promoter metal particles. Several indications for the existence and importance of hetero-bimetallic centers have been obtained by comparing activities and selectivities of mechanical mixtures of monometallic catalysts, of catalysts prepared by co-impregnation using different precursors and of mixed-metal cluster-derived catalysts. The close neighborhood of promoter metal and cobalt centers seems to be a precondition for the cooperative carbonylation and hydrogen activation, and spillover does not play an important role in the transfer of activated hydrogen. The role of the promoter metals may change with the loading. For example, ruthenium supported on active carbon showed a considerable hydrocarbonylation activity at low loadings.