4-Fluoro- and 4,4’-difluorostilbene are similar in their reactivities towards the benzoyloxy radical; a similar conclusion has been reached in respect of 4-chloro- with 4,4’-dichlorostilbene and 4-phenyl- with 4,4’-diphenylstilbene. These results have been obtained by consideration of the numbers of benzoate and phenyl end-groups in poly(methyl methacrylate) made using benzoyl peroxide in the presence of the appropriate derivative of stilbene.