Pulse radiolysis techniques have been used to determine the second order rate constants for radical-radical reactions of the parent alkyl radicals produced in neat liquid cycloalkanes of C-5 to C-10 and normal alkanes of C-6 to C-17 Iodine solutions of these compounds were used to determine the radical scavenging rate constants, the extinction coefficients of the iodine atom produced from this reaction, and the second-order rate constants for the subsequent reactions of the iodine atoms. None of the three sets of rate constants were found to be completely proportional to the inverse of the viscosity of the media, indicating that they are not solely diffusion rate-controlled. However, it was possible to describe the observed rate constants for the alkyl radical-iodine and iodine atom-atom reactions with a combination of a diffusion-controlled rate constant which scales inversely with the viscosity of the media and an activation-controlled rate constant which is the same for all media. The same procedure also worked for the alkyl radical-radical reactions except that the diffusion-controlled rate constant was further modified by a spin-steric factor. No significant differences were observed in all three sets of rate constants for the normal alkanes or the cycloalkanes at the same viscosity.