Various thiazolidine derivatives, obtained through condensation between pyridoxal or pyridoxal-5'-phosphate with several aminothiols, have been obtained and examined for their NMR spectra. For the compounds tested, we demonstrated the presence in solution of the two diasteroisomeric thiazolidines, A and C, which differ for the configuration at C2, and are interconverted via the open chain Schiff's base, with different rates, depending on the pH. The possibility that, when aminothiols react with PLP bound to the enzyme, only one of the two diasteroisomers may be formed, and the biological role of such derivatives is discussed.