The rate constants at 60-degrees-C for the reactions of the benzoyloxy radical with methyl methacrylate, the E-isomers of 2-, 3- and 4-fluorostilbene and the Z-isomer of 4-fluorostilbene have relative values of 1, 41, 17, 18 and 14 respectively. These results were obtained from work involving analyses for the benzoate and phenyl end-groups in polymers of methyl methacrylate, prepared using benzoyl peroxide suitably labelled with carbon-14 and tritium using each of the fluorostilbenes in turn as an additive in a polymerization.