The ZnO-photoanalyzed oxidation of isotactic propylene appears to be a superposition of a photocatalytic oxidation of the pigmented zones and an oxidative photochemical reaction occurring in the unpigmented zones. The photoactivity of various zinc oxides produced at high and low temperatures varies at the most within a factor of 5 and is fairly independent of the surface area of the pigment. High activation energy and low initial quantum yield have been determined measuring the appearance of carbonyl compounds. The behavior of ternary systems composed of isotactic polypropylene, zinc oxide and a typical antioxidant or hydroperoxide reducer is described. Direct interaction between the pigment and the reducing agent explains the inhibition of the oxidation of polypropylene, the inhibition of the chemical consumption of the reducer and the variations of the photoreactivities due to reheating.