The enantioselective hydrogenation of ethyl pyruvate (EtPy) on Pt-alumina (E 4759), Pt black, and Pt black + alumina (mixture) catalysts modified by dihydrocinchonidine (DHCD) in acetic acid was studied by clectrospray ionization-mass spectrometry (ESI-MS). Application of the ESI-MS technique led to the recognition of a novel-type compound, O+[Al(OAc)(2)](3) (oxonium ions). The effect of the DHCD concentration, temperature, and oxonium cations on the reaction rate and the enantioselectivity has been studied. Using the Engelhard 4759 catalyst in acetic acid under mild experimental conditions (room temperature, hydrogen pressure 1 bar, DHCD concentration 0.01 mM/L) an optical yield of 92% can be achieved. The high enantioselectivity is accompanied by the following turnovers: EtPy[DHCD > 43,000, EtPy/Pt-surface > 1000, TOF = 1-2 s(-1), and DHCD/Pt-surface ratio = 0.0072. The enantioselectivity reducing factor is identified by, ESI-MS as the gradual hydrogenation of the quinoline skeleton of DHCD that becomes more pronounced with increasing temperature and hydrogenation time. The discovery of oxonium cations, the extremely low, DHCD/Pt-surface ratio, and the new data obtained by the Pt black + alumina mixture made possible an interpretation of the mechanism of the heterogeneous enantioselective hydrogenation of alpha-ketoesters.