The pathway of the hydrogen from the gas phase to the active species on the surface of the catalyst in a stirred liquid phase batch reactor was investigated by the H/D-exchange. The reaction conditions were similar to those used in industrial nitrobenzene hydrogenation. Under equilibrium conditions (in the absence of free nitrobenzene) the hydrogen supply is not limited by the dissociation of the molecular hydrogen but by the charge transfer between adsorbed hydrogen and the catalyst. It seems likely, that after dissociation the hydrogen must be additionally activated.