The effect of the prehydrogenation on the enantioselective hydrogenation of ethyl pyruvate on the Pt-alumina catalyst (Engelhard 4759) pretreated in hydrogen at 673 K was studied by ESI-MS. It was established that in the concentration range used the hydrogenation of the quinoline skeleton of cinchonidine and dihydrocinchonidine takes place under relatively mild conditions (room temperature, 1 bar H-2 pressure), leading to a decrease in the optical yield attained (90%). In order to avoid this, it is necessary to prehydrogenate the pretreated catalyst in the absence of modifier and hydrogenation of ethyl pyruvate must be stopped approximately at 70% conversion. (C) 2002 Elsevier Science B.V. All rights reserved.