The anodic oxidation of sulfuric acid to peroxodisulfuric acid has been investigated on boron-doped synthetic diamond electrodes obtained by hot filament chemical vapor deposition. The results have shown that high current efficiency for peroxodisulfuric acid formation can be achieved in concentrated H2SO4 (7.5 mol dm(-3)) and using high current densities (200 mA cm(-2)). The main side reaction is oxygen evolution. A reaction mechanism, involving the hydroxyl radical, also has been proposed.