An ESR spectrum is observed when a sample of polyacrylonitrile powder is dissolved in a dimethyl sulphoxide solution of nitrosobenzene at room temperature. However, the spectrum is not associated with the trapping of occluded radicals from within the polymer latex, but with the hydrogen atom adduct. [Similarly, when an aqueous emulsion of poly(methyl methacrylate) is added in place of the polyacrylonitrile powder the spectrum of the hydrogen atom adduct is again observed.] Experiments in dimethyl-d6 sulphoxide indicate that the hydrogen atom does not originate from the solvent. At higher temperatures (383 K) the spectrum of a mixture of the hydrogen atom and methyl radical adducts is observed. Experiments in dimethyl-d6 sulphoxide show that the latter radical originates from the solvent.