Abstract The products of the reaction between n-butyllithium, complexed with tetramethylethylenediamine or THF, and chloro- and bromostyrene copolymers were identified by trapping the lithiated sites with 2-methyl-2-nitrosopropane. From the e.s.r. spectra of the resulting nitroxide-labelled polymers it was concluded that bromostyrene units undergo direct lithium—bromine exchange. Styrene—bromostyrene copolymers are therefore useful precursors to labelled polystyrene. Chlorostyrene units, by contrast, react with butyllithium to yield intermediate benzynes to which another molecule of butyllithium adds. The product contains a mixture of lithiated sites, each carrying a n-butyl group ortho to the lithium atom; the resulting nitroxide-labelled polymers are unsuitable for dynamic studies. The observations on halogenated styrene copolymers were complemented by experiments on small model molecules.