The flame retardant additive compositions comprise (a) one or more bromocycloaliphatic flame retardants; (b) one or more bromoaromatic flame retardants; and (c) one or more synthetic zeolites. Flame retardant polymer compositions comprising at least one thermoplastic polymer such as HIPS with which has been blended, singly and/or in one or more combinations, a flame retardant amount of at least (a), (b), and (c) have a very desirable balance of properties.