AbstractThe mechanism of the copolymerization of two components in addition‐polymerization has been examined in detail for the systems styrene‐methyl methacrylate, styrene‐butyl acrylate, and styrene‐p‐methoxystyrene. In such reactions nine elementary‐radical processes are involved. By kinetic analysis of the problem combined with measurements of the rate, composition, and molecular weight of the copolymers and measurements of the rate‐coefficients for the polymerization of individual components, all the nine rate‐coefficients may be determined with reasonable accuracy.One of the unusual factors which comes out of this study is that in the reaction of methyl methacrylate and butyl acrylate the velocity with which dissimilar radicals interact with each other is very much greater than the geometric mean reactivity of the separate radicals. On the other hand with the p‐methoxystyrene system completely normal behaviour is obtained and the rate of the interaction of dissimilar radicals is exactly equal to the geometric mean of the reactivity of the separate radicals.