Electrolysis in aq. 1M HClO4 and 1M H2SO4 solns. was carried out under galvanostatic conditions using B-doped diamond electrodes (BDD). Analyses of the oxidn. products showed that in 1M HClO4 the main reaction is oxygen evolution, while in H2SO4 the main reaction is the formation of H2S2O8. In both electrolytes small amts. of O3 and H2O2 are formed. Finally, a simplified mechanism involving hydroxyl radicals formed by H2O discharge is proposed for H2O oxidn. on B-doped diamond anodes.