The role of boron species in selective hydrogenation of o-chloronitrobenzene (o-CNB) over polymer-stabilized ruthenium colloids has been studied. Of the two kinds of boron species (boride and berate) present in ruthenium colloids prepared by NaSH4 reduction, it was verified that only berate had a significant effect on the hydrogenation of o-CNB, while boride performed as a spectator. When metal cations were added into ruthenium colloidal catalyst system as modifiers, respectively, we observed that the synergetic effect between berate and metal cations occurred in these systems. It is suggested that metal complex ions were formed between the metal cations and berate ions.