Provided are vinyl chloride polymer compositions having a composite elastomer modifier incorporated therein, whereby said polymers exhibit on excellent balance of the physical properties of thermal resistance, transparency, and impact resistance. The composite elastomer modifier is comprised of (A) 20 to 60% by weight of a graft copolymer prepared by grafting methyl methacrylate and styrene onto a butadiene rubber in a specified sequential multi-stage polymerization procedure of (B) 40 to 80% by weight of a copolymer comprised of .alpha.-methylstyrene, methyl methacrylate and acrylonitrile. The composite elastomer modifier may be prepared by either blending graft copolymer (A) with copolymer (B) or by polymerizing a monomer charge of copolymer (B) in the presence of graft copolymer (A).