A polymer mixture of(a) 45 to 50 parts by weight of a graft product of a mixture of 25 to 40 parts by weight acrylonitrile and 75 to 60 parts by weight styrene, .alpha.-methylstyrene, alkylmethacrylate or mixtures thereof on a particulate, highly crosslinked diene rubber having a rubber content of 20 to 40% by weight,(b) 45 to 10 parts by weight of a graft product of a mixture of 20 to 35 parts by weight acrylonitrile and 80 to 65 parts by weight styrene or .alpha.-methylstyrene on a particulate, highly crosslinked diene rubber having a rubber content of 50 to 75% by weight, the graft yield being greater than 60% by weight,(c) 10 to 50 parts by weight of a particulate, partially crosslinked, rubber-like copolymer of a C.sub.2 -C.sub.8 alkylacrylate and/or butadiene containing up to 40% by weight (based on copolymer) acrylonitrile which has an average particle diameter (d.sub.50) of from 0.1 to 0.3 .mu.m and a gel content of 40 to 99% by weight and(d) optionally up to 5 parts by weight of a high molecular weight, uncrosslinked copolymer of styrene or methylmethacrylate and up to 40% by weight acrylonitrile having an intrinsic viscosity [.eta.] (in dimethylformamide at 20.degree. C. of >4 dl/g,a process for its production and its use as an additive in polyvinyl chloride molding compositions.