The invention relates to a flame retardant containing a) at least one sulphur compound of formula (I), A1-( Z1)m- (S)n- (Z2)p- A2 (l) wherein the symbols and indices have the following meanings, A1, A2 are both or independently of one another C6-C12 aryl, cyclohexyl, Si(ORa)3, a saturated, partially unsaturated or aromatic, monocyclic or bicyclic ring having 3 to 12 ring members which contains one or more heteroatoms from the group N, O and S, and is unsubstituted or is substituted by one or more substituents of the group O, OH, S, SH, COORb, CONRcRd, C1-C18 alkyl, C1-C18 alkoxy, C1-C18 thioalkyl, C6-C12 aryl, C6-C12 aryloxy, C2-C18 alkenyl, C2-C18 alkenoxy, C2-C18 alkinyl and C2-C18 alkinoxy; Z1, Z2 are both or independently of one another -CO- or -CS-; Ra is C1-C18 alkyl; Rb, Rc, Rd are both or independently of one another H, C1-C18 alkyl, C6-C12 aryl or an aromatic, monocyclic or bicyclic ring having 3 to 12 ring members which contains one or more heteroatoms from the group N, O and S; m, p are both or independently of one another 0 or 1 and n is a natural number from 2 to 10, and b) at least one halogen-free organic phosphorus compound having a phosphorus content in the range from 5 to 80 wt %, based on the phosphorus compound, is suitable in particular for protection of foams based on styrene polymers.