DC corona discharges over water were studied to decompose persistent organic compounds. Since active species with short lifetimes are generated from corona discharges, they can be effectively used for purifying water. Acetic acid, a model persistent organic compound, was decomposed by the corona discharge in oxygen 4 mm above the water surface. As the voltage increased, the decomposition rate increased, which was also enhanced with increasing initial pH. Our results indicate that liquid phase reactions are important for water treatment using a DC corona discharge.