The thermal stability of poly(vinyl chloride) prepared by radiation polymerization has been examined by thermogravimetric analysis, hydrogen chloride evolution, and visible spectra measurements in films and solutions. Radiation polymerized PVC, prepared with relatively high radiation doses, is much less stable than a commercial suspension polymerization PVC. On the other hand, the stabilities of radiation polymerized PVC, prepared with relatively high radiation doses, is much less stable than a commercial suspension polymerization PVC. On the other hand, the stabilities of radiation polymerized samples, which had received less than 0.45 Mrad, are comparable with commercial PVC. It is suggested that the allylic chlorine atoms produced in the radlysis of the polymer, concurrent with its formation, are chiefly responsible for the instability of the high doses samples.