The ruthenium(II) complexes [Ru(eta(2)-P-P)(eta(6)-p-cymene)Cl]Cl, P-P = diphenylphosphinomethane (dppm), diphenylphosphinoethane (dppe), or diphenylphosphinopropane (dppp), and the highly water-soluble analogues, [Ru(eta(2)-P-P)(eta(6) -arene)Cl]Na-3, P-P = 1,2-bis(di-4-sulfonatophenylphosphino)benzene (dppbts), arene = p-cymene, benzene, or [2.2]paracyclophane, are efficient catalyst precursors for the hydrogenation of styrene in an aqueous biphase. By the use of high gas pressure NMR techniques and electrospay ionization mass spectrometry, the active species in the hydrogenation have been indirectly identified to be a dihydrogen complex, which also catalyzes H/D isotope exchange. Using the ruthenium(II) dppbts derivatives as precatalysts, evidence is provided for an arene exchange process that takes place during the catalytic hydrogenation of styrene. Together, these results lead to the proposition of a catalytic cycle for the hydrogenation of the C=C double bond of styrene using ruthenium(II)-arene diphosphine complexes.