The reactions of methyl radicals, generated by the photolysis of acetone, with cyclohexane and a number of methyl-substituted cyclohexanes have been studied. The Arrhenius parameters corresponding to overall hydrogen abstraction have been obtained from experiments in the temperature range 100–200 °C based on the value of the rate constant for recombination of methyl radicals. The overall rate constants for hydrogen abstraction by methyl radicals could be adequately expressed as the sum of contributions from each tertiary, secondary and primary hydrogen atom for all of the following nine compounds which were studied: cyclohexane, methylcy clohexane, 1,2-dimethylcyclohexane, 1,3-dimethylcyclohexane, 1,4-dimethylcyclohexane, 1,1-dimethylcyclohexane, 1,3,5-trimethylcyclohexane, 1,2,4-trimethylcyclohexane and 1,1,3-trimethylcyclohexane. The Arrhenius parameters for the abstraction of secondary hydrogen atoms are log (A/cm3 mol–1s–1)= 10.83, E/kJ mol–1= 37.4, and those for the abstraction of tertiary hydrogen atoms are log (A/cm3 mol–1 s–1)= 11.78, E/kJ mol–1= 35.7.