Activated carbons are commonly used as electrode materials in numerous applications. In this work the cathodic reduction of oxygen species formed on a carbon surface during chemical oxidation as well as electrogenerated in situ was studied at powdered activated carbon electrodes (paces). The peak current produced during the cathodic reduction of oxygen species increased significantly when anodically-treated electrode materials were used. Similarities and differences in the electrochemical behaviour of chemically and electrochemically generated oxygen species were detected cyclovoltammetrically. In both cases the bonding of oxygen with the carbon surface appears to be similar.