AbstractFlame retardant formulations for polypropylene are described that comprise a heat‐stable phosphonium bromide or phosphine oxide, containing cyanoethyl substituents, in combination with ammonium polyphosphate. At additive levels of (20–30)%, flame‐retarded polypropylene, with an Underwriters' Laboratories UL 94 V‐0 rating is obtained with little effect on thermal and mechanical properties of the polymer. A characteristic property of this flame retardant mixture is low smoke generation during burning. Mechanism studies indicate that the organophosphorus component alone acts as an inhibitor in the gas phase, whereas the ammonium polyphosphate alone acts in the condensed phase. The synergistic effect observed by the combination of these two components is due to the formation of a phosphorus‐rich char which forms an insulat