A polyblend with good impact properties, good light stability, good heat stability, low water vapor transmission and low oxygen permeability has a polymerization graft component with a rubbery substrate and a composite superstrate. Initially, the rubbery substrate is grafted with a monomer mixture containing little or no nitrile monomer and generally comprised of monomers selected from the group consisting of monovinylidene aromatic hydrocarbon, alkyl esters of unsaturated carboxylic acids, unsaturated carboxylic acids, vinyl alkanoates and vinyl ethers. Thereafter, the resultant graft copolymer is admixed and grafted with a second monomer formulation containing at least 60 percent by weight of ethylenically unsaturated nitrile monomers. The resultant composite graft copolymer is then blended with a matrix of an ethylenically unsaturated nitrile polymer to provide the desired impact modified polymer blends.