A flame retardant additive composition comprising a phosphoric acid salt selected from the group consisting of melamine phosphate, melamine pyrophosphate, melamine orthophosphate, diammonium phosphate, monoammonium phosphate, phosphoric acid amide, melamine polyphosphate, ammonium polyphosphate, polyphosphoric acid amide and combinations of two or more of the foregoing; a metal hydroxide; and an organic phosphate. The flame retardant additive is useful in a wide range of thermoplastic compositions.