The present invention relates to a moulding composition comprising (A) 30% to 85% by weight of a copolymer comprising as structural units at least one alpha, beta-unsaturated monocarbonitrile in amounts of at least 30% by weight, based on the total weight of component (A), and at least one aromatic vinyl monomer as component (A), (B) 10% to 40% by weight of a graft rubber having an average particle size of 100 to 280 nm, comprising a graft base b1, composed of at least one alpha, beta-unsaturated monocarboxylic ester, and at least one graft shell b2, composed of at least one aromatic vinyl monomer and at least one alpha, beta-unsaturated monocarbonitrile as component (B), (C) 5% to 30% by weight of a graft rubber having an average particle size of 410 to 1000 nm, comprising a graft base c1, composed of at least one alpha, beta-unsaturated monocarboxylic ester, and at least one graft shell c2, composed of at least one aromatic vinyl polymer and at least one alpha, beta-unsaturated monocarbonitrile as component (C), and (D) 0% to 10% by weight of additives as component (D), the sum of the amounts of components (A), (B), (C) and (D) being 100% by weight, it being essential to the invention that the weight ratio of component (B) to component (C) is in the range from 3:1 to 1 :1 and that the amount of the alpha, beta-unsaturated monocarbonitrile in component (B), in % by weight relative to the total weight of the graft shell b2, and in component (C), in % by weight relative to the total weight of the graft shell c2, is in each case 5 to 10 percentage points below the amount of alpha, beta-unsaturated monocarbonitrile of component (A), in % by weight relative to the total weight of component (A), and also to processes for preparing these moulding compositions, to the use of these moulding compositions for producing films, mouldings or fibres, and also to these films, mouldings or fibres themselves.