Samples of trans-stilbene and trans- p-fluorostilbene labelled with 14C have been used for measurement of the small extents to which they copolymerize with styrene during radical polymerization. Copolymers of styrene with p-fluorostilbene have been examined by 19F n.m.r.; there are important differences between the spectra for products made using benzoyl peroxide and azobisisobutyronitrile as initiators. The results are explained in terms of a high probability of a stilbene unit being adjacent to the ester end-group derived from an aroyloxy radical.