We have investigated the electrochemical oxidation of sulfuric acid on boron-doped synthetic diamond electrodes (BDD) obtained by HF CVD on p-Si. The results have shown that high current efficiency for sulfuric acid oxidation to peroxodisulfuric acid can be achieved in concentrated H2SO4 ( > 2 M) at moderate temperatures (8-10 degreesC). The main side reaction is oxygen evolution. Small amounts of peroxomonosulfuric acid (Caro's acid) have also been detected. A reaction mechanism involving hydroxyl radicals, HSO4- and undissociated H2SO4 has been proposed. According to this mechanism electrogenerated hydroxyl radicals at the BDD anode react with HSO4- and H2SO4 giving peroxodisulfate.