The results of a study of the oxidation peak observed with InP electrodes in the negative potential region are reported. The study was performed with the following three InP electrodes in 0.1M KOH : n-InP (5 X 10(15) cm-3), n-InP(4 x 10(18) cm-3), and p-InP (4 x 10(18) cm-3). n-InP electrodes having low carrier concentration exhibited a photovoltage associated with the oxidation peak. These observations signify that InP plays an active role in this oxidation process and forms a major component of the peak. X-ray photoemission analyses indicate that the oxidation product of InP is InPO4. Comparison of the voltammograms of InP with those of metallic indium indicate the possible presence of indium oxide. These observations suggest that the oxidized surface is composed primarily of InPO4 with minor amounts of In(OH)3.