The formation of longer polyene chains (thermal discoloration) has been found to be avoidable by using thiol-s-triazines, such as 2-anilino-4,6-dithiol-s-triazine (AF), which hardly reacts with the original chlorine atoms but tends to replace the activated chlorine (allylic chloride) that plays an important role in triggering zipperlike dehydrochlorination. The optimum stabiliizing system is: PVC, 100; DOP, 50; AF, 0.18; Zn stearate, 0.5; Ba stearate, 1.5 parts.