Mesoxalic acid (ketomalonic acid) was prepared by oxidative dehydrogenation with air of tartronic acid in aqueous solution on carbon-supported platinum-bismuth catalysts. By increasing the pH, using a higher catalyst/substrate ratio or increasing the temperature, higher yields could be obtained (maximum yield obtained of 65% at 80% conversion). The results obtained for this reaction and for the analogous catalytic oxidations of glyceric acid to hydroxypyruvic acid, and lactic acid to pyruvic acid, enabled a general reaction mechanism to be proposed for the selective oxidation of alpha-hydroxy acids to alpha-keto acids on platinum-bismuth catalysts.