The effect of oxygen and catalyst reduction temperature in enantioselective hydrogenation of 1-phenyl-1,2-propanedione over commercial Pt/Al2O3 catalyst was investigated. Dichloromethane was used as solvent. The catalyst was modified in situ with (-)-cinchonidine. Relatively high enantiomeric excesses (65%) of (R)-1-hydroxy-1-phenyl-2-propanone were obtained with the solvent used as received, i.e. containing traces of dissolved oxygen and other impurities. Dichloromethane dissociated partially on the Pt/Al2O3 surface causing desorption of methane, ethene and HCl from the catalyst during TPD according to mass spectrometric analysis. Under anaerobic conditions the reaction rate was low giving only about 40% enantiomeric excesses of (R)-1-hydroxy-1-phenyl-2-propanone. When injecting 5 mm(3) of oxygen into the reactor a beneficial effect was observed (i.e. higher reaction rate and enantiomeric excess) in comparison with anaerobic conditions. Poisoning effect of oxygen was observed when injecting 500 mm(3) of oxygen into the reactor. Effect of catalyst reduction temperature was studied at three different temperatures (170, 400 and 455 degrees C). Highest reaction rates and enantiomeric excesses were obtained with the catalyst reduced at 400 degrees C. Methane was desorbed from the catalyst at temperatures between 263 and 383 degrees C which could be the explanation for the lower activity of the catalyst reduced at 170 degrees C. It was demonstrated that small amounts of oxygen can have a beneficial effect in enantioselective hydrogenation of 1-phenyl-1,2-propanedione and also that catalyst reduction temperature plays an important role in obtaining high enantiomeric excesses.