The carbonyls of the various benzoate end groups in poly(methyl methacrylate-co-styrene) prepared with benzoyl-carbonyl-13C peroxide give rise to discrete and characteristic signals in the 62.9 MHz13C NMR spectra of the copolymers. Thus, by integration of the spectra, it is possible to estimate the relative reactivity of benzoyloxy radicals towards styrene and methyl methacrylate. In addition, information on the amount of initiator consumed in other processes such as transfer to initiator and primary radical termination can be obtained.