The electrochemical oxidation of cyclic amino acids such as histidine and tyrosine has been studied in a strongly basic solution by means of an in-situ FTIR spectroscopic method. The terminal COO- group of fully unprotonated anions was adsorbed on a platinum electrode from + 0.4 V versus Ag \ AgCl \ sat KCl, and the concentration of the adsorbent increased with increasing potential. However. the adsorption strength of the terminal carboxyl was weakened at + 1.0 or + 1.1 V for histidine or tyrosine, respectively. In the oxidation of tyrosine, the benzene ring started to be oriented in parallel to the electrode surface at + 0.3 V and the flat orientation reached completion at + 0.6 V. Histidine and tyrosine were both oxidized from + 1.1 V in a NaOH solution of pH 13.