Abstract: Polymer composition comprising a copolymer of a vinylaromatic compound, a vinyl cyanide and, optionally, a third monomer which copolymer is grafted on a rubber, a flame-retardant phosphorus compound and a charforming component, characterised in that the polymer composition also comprises a polymer P which comprises 1-100 wt. % monomeric units with the following structure, the wt. % being relative to said polymer P ##STR1## where m=1-5. This polymer composition shows improved flame-retardant properties as a result of synergism with respect to the flame-retardant properties between the charformer and polymer P.