Polyolefin compositions comprising a particular hindered phenolic antioxidant selected from the group consisting of tris(3,5-di-tert-butyl-4-hydroxybenzyl)isocyanurate, 1,3,5-tris-(3,5-di -tert-butyl-4-hydroxybenzyl)-2,4,6-trimethylbenzene and the calcium salt of the monoethyl ester of 3,5-di-tert-butyl-4-hydroxybenzylphosphonic acid, an oligomeric sterically hindered amine light stabilizer, a hydroxylamine stabilizer or trialkylamine N-oxide stabilizer, and, optionally, an organic phosphorus stabilizer, are effectively stabilized against degradation upon melt processing, long term heat aging, and exposure to combustion products of natural gas.