IrO2 and boron-doped diamond electrodes were tested by preparative electrolysis of simple model compds. in acid medium. In oxidn. reactions of more complex mechanism than simple electron transfer, these electrodes are active only at potentials of simultaneous oxygen evolution. At IrO2 electrodes, the reaction is mediated by chemisorbed oxygen, probably forming IrO3-type active sites, and yield highly selective oxidn. products at low overpotentials and with moderate current efficiency. At diamond electrodes, mediation at high overpotentials is due to physisorbed OH radicals leading to complete combustion of the org. compd. with high current efficiency. [on SciFinder (R)]