Thermoplastic compositions comprising polyacrylonitriles that contain greater than 90% acrylonitrile as monomer, or copolymers of such polyacrylonitriles with an olefinic unsaturated co-monomer, can be formed by the use of certain compatibilizers and/or thermal stabilizers. Other thermoplastic compositions comprising the high nitrile polyacrylonitriles can be obtained by melt blending the polyacrylonitriles with certain aromatic engineering thermoplastic polymers in the presence of compatibilizers and thermal stabilizers. All of the foregoing high nitrile compositions have been found to possess good optical and gas barrier properties, and good stress crack resistance.