Electrochemical oxidation of diglycine (G(2)), triglycine (G(3)) and tetraglycine (G(4)) has been studied in an alkaline medium by means of in situ FTIR spectroscopic and electrochemical quartz crystal microbalance (EQCM) methods. The adsorption of the terminal carboxyl group of these peptides occurred around + 0.3 V versus Ag \ AgCl, and the oxidative decomposition was observed from ca. + 1.0 V. A striking difference in the oxidation of amino acid and peptides was that the former reaction continued to proceed with increasing potential but the oxidation of peptides was rather restrained at high positive potentials. The major reason for such constraint was that the glycine or glycil-glycine generated in the anodic oxidation of peptides was in the acid form and its adsorption and following oxidation on the electrode became considerably difficult. Formation of the acid form of glycine or glycil-glycine was attributed to the lowering of pn which occurred because oxygen evolution occurs in further positive polarization. The EQCM data indicated that the adsorption of glycil-peptide occurred from ca. + 0.2 V, the adsorbed carboxyl group was decomposed oxidatively at ca. + 1.1 V and a part of the remaining adsorbents was detached simultaneously from the electrode with the evolution of oxygen in further positive polarization.