The radiolytic decomposition of the B-group vitamins in 0.1 mM neutral aqueous solutions has been studied in oxygenated as well as N2O saturated solutions. In the former G(-Vit) and G(H2O2) values suggested the occurrence of reaction between the vitamin radioals and the H2O2formed during radiolysis in the oase of pyridoxin, folic acid and cyanocobalamin. The occurrence of this reaction was directly confirmed in N2O saturated solutions initially containing hydrogen peroxide. In this case an increased vitamin decomposition and a parallel consumption of the H2O2were observed. Reactivities of different types of radicals with H202are discussed.