Stabilized flame retardant additives and their use

Flame retardant additive compositions having enhanced thermal stability are described. They comprise a blend formed from (A) at least one organic bromine flame retardant selected from (i) a flame retardant compound having a plurality of bromine atoms directly bonded to a cycloaliphatic ring, (ii) the bis(2,3-dibromopropyl ether) of tetrabromobisphenol-A, and (iii) the bis(2,3-dibromopropyl ether) of tetrabromobisphenol-S, (B) an alkyltin mercaptoalkanoate, and (C) a zeolite adjuvant. The proportions are 0.01-0.08 part by weight of (B) per part by weight of (A), and 0.01-0.35 part by weight of (C) per part by weight of (A). Thermoplastic polymer compositions in which these additive compositions are employed possess enhanced properties.