The transition state for the metal ligand bifunctional addition step in Noyori's enantioselective ketone hydrogenation was investigated using intramolecular trapping experiments. The bifunctional addition between the Ru dihydride trans-[Ru((R)-BINAP)(H())2((R,R)-dpen)] and the hydroxy ketone 4-HOCH2C6H4(CO)CH3 at -80 degrees C exclusively formed the corresponding secondary ruthenium alkoxide trans-[Ru((R)-BINAP)(H)(4-HOCH2C6H4CH(CH3)O)((R,R)-dpen)]. Combined with the results of control experiments, this observation provides strong evidence for the formation of a partial Ru-O bond in the transition state.