AbstractThe tritium fractionation between water and radicals formed in the radiolysis of dilute solutions of monomerous methyl methacrylate in H2O‐HTO and D2O‐DTO mixutres was studied. A parallel determination of the tritium content in molecular hydrogen was performed. Also, the isotopic composition of the initial molecular hydrogen was measured in the concentration range 10 to 90 Mol% D2O of H2O‐HDO‐D2O mixtures.Hydrogen atoms, hydroxyl radicals and molecular hydrogen produced in the radiolysis of these solvents were found to be depleted in heavier hydrogen isotopes. The isotope effects on the composition of hydrogen atoms are discussed in terms of the rate isotope effects in proton transfer. The isotope effects on the composition of hydroxyl radicals are close to those on molecular hydrogen and their extent suggests ionic dissociation of water as a rate‐determining step in the process of formation of both