Platinum sols, prepared from H2PtCl6 in aqueous methanol, are, when modified by cinchona alkaloids, highly reproducible catalysts for the enantioselective hydrogenation of ethyl pyruvate. It is shown that the presence of HCl in the as-prepared sols has a marked effect on rate and reproducibility, and that removal of HCI by dialysis gives catalysts which display exceptional reproducibility and higher rates. The reliability of these colloidal catalysts will allow their use as a reliable test system both for precise rate studies, in the absence of possible support effects, of the various reaction parameters which affect catalyst performance in this reaction, and also for screening of alternative modifiers to cinchona alkaloids.