Abstract The changes in mechanical properties of processed high density polyethylene, when “u.v. stabilizers” are added, are examined using tensile and melt flow measurements. The results show that, at low additive concentrations, antioxidant effects predominate; at high concentrations, a reinforcement effect is observed. Degradation measurements show that the u.v. stabilizers behave as excellent oxidation inhibitors in the dark; this finding has implications regarding their mechanistic action.