The present invention provides a thermoplastic composition which may be unvulcanized or vulcanized. The thermoplastic composition can be a blend of a thermoplastic engineering resin, a halogenated terpolymer of a C.sub.4 to C.sub.7 isomonoolefin, a para-alkylstyrene, and a multiolefin. The engineering resin can be a polyamide, such as nylon 6/66 copolymers for example. The isomonoolefin can be an isobutylene for example. The multiolefin can be a C.sub.4 to C.sub.14 diene, such as isoprene for example.