A thermoplastic molding composition that features constancy of properties, especially toughness and processability is disclosed. The composition contains a first graft rubber polymer that is a product of emulsion polymerisation of styrene and acrylonitrile in the presence of a butadiene polymer latex (A) with a mean particle diameter d50 of 80 to 220 nm, a second graft rubber polymer obtained by emulsion polymerisation of styrene and acrylonitrile in the presence of a butadiene polymer latex (B) with a mean particle diameter d50 of 340 to 480 nm, and a rubber-free copolymer of styrene and acrylonitrile. Importantly, the butadiene polymer latex (A) is the product of seed polymerization using a seed latex with a mean particle diameter of 10 to 100 nm, and the butadiene polymer latex (B) is the product of seed polymerisation using the butadiene polymer latex (A) as seed latex.