We studied the riboflavin-sensitized photodecomposition of ascorbic acid by visible light using the polarographic technique. The reaction was carried out at 25-degrees-C in the pH range 2.5-6.0 under anaerobic and aerobic conditions and the reaction rate was found to depend on the pH in both cases. Oxygen was consumed during photooxidation and the rate was limited by the riboflavin concentration. Formation of hydrogen peroxide appeared under both aerobic and anaerobic conditions. Reaction steps related to the formation of hydrogen peroxide were included in the reaction mechanisms.