The electron spin resonance of polyvinyl chloride (PVC) irradiated in NH3 was studied. The radicals produced on irradiation in NH3 do not differ greatly from those obtained on irradiation in vacuum, but the reactivity of the former to SO2 to form RSO2ċ radicals is far higher than that of those obtained in vacuum. The radicals produced by irradiation at −78°C. and in vacuum was also studied. The spectra of these radicals were different from those of the radicals produced at 20°C. and in vacuum. The reactivity of the radicals produced in PVC is interpreted on the basis of the length of double bonds conjugating with the radical π-electron.