Fluoromethylglyoxal in the presence of glutathione has been shown to undergo a novel glyoxalase I-catalyzed product partitioning to S-fluorolactoylglutathione and S-pyruvylglutathione with fluoride elimination. While the partition ratio (fluoride eliminated/total fluoride) was insensitive to pH 5.5-7.5 and concentrations of substrate and glyoxalase I, it was species-dependent. When [1-2H]fluoromethylglyoxal was reacted with glyoxalase I, an increase in the partition ratio and retention of deuterium in the product fluorolactate was observed. This result can only be explained by a selective primary isotope effect on the protonation of an enediol intermediate relative to fluoride elimination.