The production of H(2)O(2) as an environmentally benign chemical was attempted using underwater electrical discharge with ultrasonic irradiation to examine the synergic effects of ultrasound and plasma. The results showed that H(2)O(2) production was reduced, while the production efficiency based on the energy consumption for the discharge was improved. The study suggested that ultrasonic irradiation may be an effective method to intensify processes in underwater electrical discharge.