The photooxidation of PVC was followed by IR titration of C˭O groups. The kinetics of formation of these groups is characterized by an initial autocatalytic behavior. This feature disappears after a few hundred hours of exposure, and a marked decrease of the rate of reaction is then observed. We have shown that no relation exists between these variations of the reaction rate and the variation of UV absorption of films. HC1 is formed in the reactions of photooxidation and photoelimination. The auto-catalysis and the autoinhibition are favored when the accumulation of HC1 is itself favored.