The isotopic composition of hydrogen evolved in the radiolysis of C6D12–C6H12mixtures has been measured in the presence of various amounts of benzene and iodine. The yield of the unimolecular decomposition and the specific yield of the bimolecular reaction are reduced in approximately the same proportion as the yield of total hydrogen. It is concluded that (i) iodine and benzene interact to a comparable extent with both unimolecular and bimolecular decomposition modes, (ii) it is probable that neither of these additives act by scavenging, and (iii) it is probable that they act by quenching a common precursor of the two decomposition modes.