AbstractVarious PVC films containing a thermal stabilizer selected from three kinds of metal stearates and three kinds of organotin compounds were irradiated with ultraviolet rays and the unirradiated samples were subjected to the ESR analysis. Further, these PVC films were subjected to the photodegradation test using a weatherometer at a sample surface temperature of 0°C. By tracing the conversion of methyl radicals to alkoxy radicals or peroxy radicals with the lapse of time according to the ESR, effects of the foregoing stabilizers for stabilizing PVC against actinic rays could be evaluated. Results of the ESR were well in agreement with the results obtained at the photodegradation test using the weatherometer