AbstractThe effect of thermal‐oxidative processing on the stability to ultraviolet irradiation of high density polyethylene containing a catechol hindered‐phenyl phosphite has been studied by periodic observation of carbonyl index by infrared spectrometry and by recording embrittlement times.Certain of the possible thermal‐oxidative reaction products of catechol hindered‐phenyl phosphite, catechol hindered‐phenyl phosphate and mono‐(σ‐hydroxyphenyl)phosphoric acid have also been studied. The results are consistent with the theory previously developed that the latter or its dehydration product, catechol hydroxy phosphate, is the effective antioxidant produced dur