The radiation chemistry of CH3I in 3-methyl pentane has been examined at 77 K in order to understand the mechanism of methyl radical decay. It was observed that for the same concentration of CH3I and equal γ dose, quickly frozen solutions gave 12 to 15% higher yields of CH3radicals and CH4as compared to slowly frozen solutions. In the latter caseG(12) was found to be higher. The differences in the yields of products have been attributed to effects caused by aggregation of solute molecules while the observed 1st order decay of CH3radicals have been attributed to its recombination with iodine atoms. The relative probability of hydrogen atom abstraction from matrix molecules as compared to homolytic dissociation of hot alkyl radicals produced by γ-radiation has been estimated in a few cases.