AbstractThe light‐stability of chlorinated poly(vinyl chloride) (PVCC) is investigated and the quantum yields of the major process involved are determined. By UV exposure, PVCC undergoes a fast dehydrochlorination process which develops ten times more efficiently than with poly(vinyl chloride) (PVC), leading to long chlorinated polyene sequences which are responsible for the rapid coloration of the irradiated polymer. Chain scission and crosslinking occur simultaneously, both in the presence and in the absence of oxygen, indicating that UV radiations are able to cleave the polymer chains. By reaction with polyenyl radicals, oxygen reduces slightly the efficiency of the dehydrochlorination, while promoting at the same time the formation of carbonyl and hydroperoxide groups in the polymer chain. The reasons of the high instability of PVCC against UV light are discussed and an overall reaction scheme is suggested to account for the various processes observed and their relative efficiencie