The influence of substituents on the additive reactivity is reconsidered with particular reference to the Arrhenius parameters. Using the correction terms for the frequency factor in order to account for correct orientation of reactants, the activation energy of ethylene and a number of ethylene derivatives have been calculated. The correlation with the bond stretching force constant and the grouping together of ethylene derivatives on the basis of mechanistic differences follows from these results. The mutual action of groups and the peroxide effect are briefly considered. The solvent affects essentially the activation energy of the rate determining step.