Chemiluminescence of ascorbic acid in Fenton solution was investigated at acid concentration from 10(-7) to 1 mol/l. The yield of chemiluminescence exceeds spontaneous light from Fenton solution at [Asc] - 10(-4) - 10(-3) mol/l in the presence of dissolved oxygen, and prooxidant properties are displayed. When there is no oxygen in solution, this effect is not observed. A comparison of experimental data and oxidation process simulation shows that ascorbic acid is an antioxidant. Its transformation products in the presence of oxygen exert prooxidant properties.