The addition of methyl radicals to ethylene and its homologues in
iso
-octane solution was studied over the temperature range 55 to 85°C. The relative rate constants of addition to ethylene, propylene and
iso
-butene determined at 65°C are 34, 22 and 36 respectively. Taking into account the statistical factor of 2 for ethylene, we conclude that the increasing ease of addition for this series of olefines reflects the increasing stability of the produced radicals. The rates of addition to
trans
- and
cis
-butene-2 are significantly lower (6.9 and 3.4 respectively), indicating a steric hindrance resulting from the presence of a methyl group on the carbon atom on which the reaction takes place. Identical rates of addition were found for propylene, butene-1,pentene-1, 3 methyl butene-1, heptene-1, decene-1 and hexadecene-1, indicating that the rates of addition are not affected by the length and the shape of the hydrocarbon ‘tail’. The rates of abstraction of hydrogen atoms by methyl radicals were determined. It was found that the rate constants for active hydrogen (
α
to C=C double bond) fall into three distinctive classes characterizing the primary, the secondary, and the tertiary hydrogen atoms.