AbstractThe Mayo and Lewis reactivity ratio concept was found to be applicable to the treatment of the data obtained from base‐catalyzed copolymerization studies involving eight cyclic siloxane monomers. The cyclics investigated were hexamethylcyclotrisiloxane, hexaphenylcyclotrisiloxane, and all six members of the series, where R is methyl and/or phenyl. The products of the reactivity ratios for a number of the monomer pairs studied were close to unity, thus indicating little catalyst specificity. Polymerization reactivity increased with increased ring strain and increased degree of phenyl substitution. The tetraphenyl silethylene‐siloxane cyclic, the most reactive cyclic to base‐catalyzed polymerization, was found to be about one hundred and sixty times more reactive than the least reactive cyclic, hexamethylcyclotrisiloxane. Reactivity ratios were found to be quite temperature‐dependent, thus suggesting rather large differences in activation energy. With increasing temperature, the reactivity ratios approached a value o