Polyolefins, especially polypropylene, can be made flame retardant by the incorporation therein of a synergistic mixture of a selected hindered amine substituted by an alkoxy, cycloalkoxy or hydroxyalkoxy moiety and a selected organic or inorganic brominated and/or phosphorus containing flame retardant, such as ammonium polyphosphate or decabromodiphenyl oxide wherein the amount of organic or inorganic flame retardant required to achieve an acceptable level of flame retardancy is significantly reduced compared to that needed when the hindered amine is not present.