Abstract The effect of cumene on the hydrogen yields from methylcyclohexane γ-irradiated at 100, 20, −72, −120 and −196°C has been investigated. Cumene did not compete with N2O for capturing electrons in methylcyclohexane and the yield of thermal H atoms in methylcyclohexane, determined by use of ethylene as an H atom scavenger, was smaller than the decrease in hydrogen yield in methylcyclohexane-cumene systems. Therefore, the decrease in hydrogen yield in methylcyclo-hexane-cumene systems can be ascribed mainly to positive charge transfer from methylcyclohexane to cumene. Further, the evidence of charge transfer from methylcyclohexane to cumene was found by the optical absorption method. The effect of cumene on the hydrogen yield from methylcyclohexane was similar in the temperature range from 100 to −196°C, especially at high cumene concentration. The effect of an electron scavenger at high concentrations was also examined with n-butyl chloride. In the present work, we have found that although the effect of a positive ion scavenger on the hydrogen yield from methylcyclohexane in the liquid state differs little from that in the glassy state, the effect of electron scavengers on the former differs considerably from that on the latter.