The γ-ray oxidation of 2-methyl tetrahydrofuran proceeds with a chain reaction giving an organic hydroperoxide as the major product at low doses. The rate of peroxide formation is given byand the rate constant ratio for the chain propagation to the chain termination is given byat room temperature. An ionic chain termination by O2- was found to be important only in aqueous solutions but not in pure MTHF or mixtures of cyclohexane + MTHF where the low dielectric constant of the medium seems to inhibit the O2- formation in appreciable yield.