Depending on the reaction medium ( toluene or acetic acid) striking differences were observed in the development of enantiomeric excess with time-on-stream in continuous enantioselective hydrogenation of ethylbenzoylformate on cinchonidinemodified Pt/Al2O3, which correlated well with the amount of adsorbed cinchonidine on alumina and silica from acetic acid and toluene solutions. Maximally 93% enantiometric excess (ee) of (R)-ethyl mandelate was obtained.