Oxygen-containing functional groups can be introduced onto the surface of polycrystalline boron-doped diamond electrodes by either anodic polarization or oxygen plasma treatment. Of these, the carbonyl groups are of particular interest and can be studied specifically by means of specific chemical modification with dinitrophenylhydrazine (DNPH). The modification of the surface carbonyl groups with DNPH retards the Fe2+/3+ redox reaction, which is known to be catalyzed by carbonyl groups. Anodic polarization generated a larger number of carbonyl groups per unit area that were reactive with DNPH than that generated by oxygen plasma treatment. The molar ratio of the DNPH-reactive carbonyl groups to the total of all types of oxygen atoms could be as high as 5% for electrochemically oxidized diamond surfaces. This value is reasonable in view of steric limitations. The total number of carbonyl groups per unit area introduced by oxygen plasma treatment was probably larger than that introduced by anodic polarization, but the number that can react with DNPH is lower due to disordering by energetic oxygen ions.