Abstract: A flame retardant polymer composition consists of a) 20–60 percent by weight of a thermoplastic and/or cross-linked or cross-linkable elastomer and b) as a flame retardant agent 40–80 percent by weight either of an aluminum hydroxide with the material values specific surface according to BET 3–5 mg2/g mean grain size d50 1.0–1.5 ?m residual moisture 0.1–0.4% oil absorption 19–23% water absorption 0.4–0.6 ml/g or of an aluminum hydroxide with the material values specific surface according to BET 5–8 m2/g mean grain size d50 0.8–1.3 ?m residual moisture 0.1–0.6% oil absorption 21–25% water absorption 0.6–0.8 ml/g. A process of producing the flame retardant agent is characterised in that a filter-moist aluminum hydroxide obtained by precipitation and filtration with a mean grain size of 0.8 to 1.5 ?m is subjected to mill drying in a turbulent hot air stream in such a way that, while the grain distribution is largely retained, the BET surface is increased by at least 20%.