A flame retardant comprising a) at least one sulfur compound of the formula A 1 -(Z 1) m - n -(Z 2) p -A 2 where the definitions of the symbols and indices are as follows: A 1 and A 2 are identical or different, being C 6 -C 12 -aryl, cyclohexyl, Si(OR a) 3, a saturated, partially unsaturated, or aromatic, mono- or bicyclic ring having from 3 to 12 ring members and comprising one or more heteroatoms from the group of N, O, and S, where the system is unsubstituted or has substitution by one or more substituents of the group of 0, OH, S, SH, COOR b, CONR c R d, C 1 -C 18 -alkyl, C 1 -C 18 -alkoxy, C 1 -C 18 -thioalkyl, C 6 -C 12 -aryl, C 6 -C 12 -aryloxy, C 2 -C 18 -alkenyl, C 2 -C 18 -alkenoxy, C 2 -C 18 -alkynyl, and C 2 -C 18 -alkinoxy; Z 1 and Z 2 are identical or different, being -CO-or -CS-; R a is C 1 -C 18 -alkyl; R b, R c, and R d are identical or different, being H, C 1 -C 18 -alkyl, C 6 -C 12 -aryl, or an aromatic, mono- or bicyclic ring having from 3 to 12 ring members and comprising one or more heteroatoms from the group of N, 0, and S; m and p are identical or different, being 0 or 1, and n is a natural number from 2 to 10, and b) at least one halogen-free organophosphorus compound having phosphorus content in the range from 5 to 80% by weight, based on the phosphorus compound, in an a:b ratio by weight of from 1:10 to 10:1. a) at least one sulfur compound of the formula A 1 -(Z 1) m - n -(Z 2) p -A 2 where the definitions of the symbols and indices are as follows: A 1 and A 2 are identical or different, being C 6 -C 12 -aryl, cyclohexyl, Si(OR a) 3, a saturated, partially unsaturated, or aromatic, mono- or bicyclic ring having from 3 to 12 ring members and comprising one or more heteroatoms from the group of N, O, and S, where the system is unsubstituted or has substitution by one or more substituents of the group of 0, OH, S, SH, COOR b, CONR c R d, C 1 -C 18 -alkyl, C 1 -C 18 -alkoxy, C 1 -C 18 -thioalkyl, C 6 -C 12 -aryl, C 6 -C 12 -aryloxy, C 2 -C 18 -alkenyl, C 2 -C 18 -alkenoxy, C 2 -C 18 -alkynyl, and C 2 -C 18 -alkinoxy; Z 1 and Z 2 are identical or different, being -CO-or -CS-; R a is C 1 -C 18 -alkyl; R b, R c, and R d are identical or different, being H, C 1 -C 18 -alkyl, C 6 -C 12 -aryl, or an aromatic, mono- or bicyclic ring having from 3 to 12 ring members and comprising one or more heteroatoms from the group of N, 0, and S; m and p are identical or different, being 0 or 1, and n is a natural number from 2 to 10, and A 1 and A 2 are identical or different, being C 6 -C 12 -aryl, cyclohexyl, Si(OR a) 3, a saturated, partially unsaturated, or aromatic, mono- or bicyclic ring having from 3 to 12 ring members and comprising one or more heteroatoms from the group of N, O, and S, where the system is unsubstituted or has substitution by one or more substituents of the group of 0, OH, S, SH, COOR b, CONR c R d, C 1 -C 18 -alkyl, C 1 -C 18 -alkoxy, C 1 -C 18 -thioalkyl, C 6 -C 12 -aryl, C 6 -C 12 -aryloxy, C 2 -C 18 -alkenyl, C 2 -C 18 -alkenoxy, C 2 -C 18 -alkynyl, and C 2 -C 18 -alkinoxy; Z 1 and Z 2 are identical or different, being -CO-or -CS-; R a is C 1 -C 18 -alkyl; R b, R c, and R d are identical or different, being H, C 1 -C 18 -alkyl, C 6 -C 12 -aryl, or an aromatic, mono- or bicyclic ring having from 3 to 12 ring members and comprising one or more heteroatoms from the group of N, 0, and S; m and p are identical or different, being 0 or 1, and R b, R c, and R d are identical or different, being H, C 1 -C 18 -alkyl, C 6 -C 12 -aryl, or an aromatic, mono- or bicyclic ring having from 3 to 12 ring members and comprising one or more heteroatoms from the group of N, 0, and S; m and p are identical or different, being 0 or 1, and n is a natural number from 2 to 10, and b) at least one halogen-free organophosphorus compound having phosphorus content in the range from 5 to 80% by weight, based on the phosphorus compound,