Abstract: Flame retardants are described, comprising (A) at least one metal or metalloid salt of a phosphonic acid of formula (I) wherein R is hydrogen, C1-C18alkyl, C5-C6cycloalkyl, C2-C6alkenyl, C6-C10aryl or C7-C11aralkyl and R? is hydrogen, C1-C8alkyl, C6-C10aryl or C7-C11aralkyl, the substituents R and R? that are other than hydrogen being unsubstituted or substituted by halogen, hydroxyl, amino, C1-C4alkylamino, di-C1-C4alkoxy, carboxy or C2-C5alkoxycarbonyl; and the metal or metalloid is one from Group IA, IB, IIA, IIB, IA, IVA, VA or VII of the Periodic Table; and (B) at least one flame retardant component based on a nitrogen compound, and also the use thereof in synthetic polymers.