We have studied the reactivity of glycolaldehyde (GLA) with N-acetyl-Cys and N-acetyl-Phe-Lys at physiological conditions of pH and temperature. The reaction between the N-Ac-Phe-Lys and GLA was studied in the presence of NaCNBH3 and then by using high-performance liquid chromatography (HPLC)-UV/Vis. The reaction between N-Ac-Cys and GLA was followed by stopped-flow spectroscopy with UV/Vis detection. Both the reduced Schiff base and thiohemiacetal were identified by 1H-NMR and HPLC-mass spectrometry detection. The kinetic rate constant for the thiohemiacetal formation is four orders of magnitude higher than that for the Schiff base formation. This result suggests that the thiol group represents the most important target in protein glycation.