The epimerization of the potassium salts of L-arabinonic, D-ribonic, D-lyxonic and D-xylonic acids in aqueous alkali was followed kinetically by gas Chromatographie analyses of the silylated samples. Mass spectrometric measurements indicated that the reaction proceeded by a complete proton exchange at C2, the kinetic deuterium isotope effect, k-H/k-Dbeing 4.2. Most probably the proton transfer from the C2 atom to hydroxiae ion is the rate-limiting stage of the epimerization.