Flame retardants are described, comprising A, at least one organic phosphonate of formula (I), wherein R is hydrogen, C1-C20alkyl, unsubstituted or C1-C4alkyl-substituted phenyl or naphthyl, R′ is hydrogen, C1-C20alkyl, unsubstituted or C1-C4alkyl-substituted phenyl or naphthyl or M1r+/r, n is an integer from 0 to 6, M1r+ is an r-valent metal ion or the ammonium ion, r is an integer from 1 to 4, R13 is isopropyl, isobutyl, tert-butyl, cyclohexyl or is cycloalkyl substituted by from 1 to 3 C1-C4alkyl groups, R14 is hydrogen, C1-C4alkyl, cycloalkyl or is cyclohexyl substituted by from 1 to 3 C1-C4alkyl groups, and R15 is hydrogen C1-C18alkyl, trimethylsilyl, benzyl, phenyl or sulfonyl; and B, at least one flame retardant component based on a nitrogen compound, and also the use thereof in synthetic polymers.