A process for the preparation of a graft polymer A) having bimodal particle size distribution made from, based on A), a1) from 40 to 90% by weight of an elastomeric particulate graft base a1), obtainable by polymerization of, based on a1), a11) from 75.5 to 89.5% by weight of at least one conjugated diene, a12) from 10.5 to 24.5% by weight of at least one vinylaromatic monomer and a2) from 10 to 60% by weight of a graft a2) made from, based on a2), a21) from 65 to 95% by weight of at least one vinylaromatic monomer, a22) from 5 to 35% by weight of acrylonitrile, and a23) from 0 to 30% by weight of at least one other monoethylenically unsaturated monomer, the process comprising steps (i), (ii) and (iii): (i) emulsion polymerization of monomers a11) and a12) to obtain a graft base a1), (ii) partial agglomerization of the graft base a1), and (iii) emulsion polymerization of graft monomers a21), a22) and optionally a23) onto the graft base obtained in step (i) or (ii), wherein in step (i) the graft base a1) is obtained by emulsion polymerization of an initial seed of from 3 to 8 % by weight of vinylaromatic monomer a12), - each based on a1) - followed by addition and statistical copolymerization of the remaining vinylaromatic monomer a12) together with the entire amount of conjugated diene a11). a1) from 40 to 90% by weight of an elastomeric particulate graft base a1), obtainable by polymerization of, based on a1), a11) from 75.5 to 89.5% by weight of at least one conjugated diene, a12) from 10.5 to 24.5% by weight of at least one vinylaromatic monomer and a11) from 75.5 to 89.5% by weight of at least one conjugated diene, a12) from 10.5 to 24.5% by weight of at least one vinylaromatic monomer and a2) from 10 to 60% by weight of a graft a2) made from, based on a2), a21) from 65 to 95% by weight of at least one vinylaromatic monomer, a22) from 5 to 35% by weight of acrylonitrile, and a23) from 0 to 30% by weight of at least one other monoethylenically unsaturated monomer, the process comprising steps (i), (ii) and (iii): (i) emulsion polymerization of monomers a11) and a12) to obtain a graft base a1), (ii) partial agglomerization of the graft base a1), and (iii) emulsion polymerization of graft monomers a21), a22) and optionally a23) onto the graft base obtained in step (i) or (ii), wherein in step (i) the graft base a1) is obtained by emulsion polymerization of an initial seed of from 3 to 8 % by weight of vinylaromatic monomer a12), - each based on a1) - followed by addition and statistical copolymerization of the remaining vinylaromatic monomer a12) together with the entire amount of conjugated diene a11). a21) from 65 to 95% by weight of at least one vinylaromatic monomer, a22) from 5 to 35% by weight of acrylonitrile, and a23) from 0 to 30% by weight of at least one other monoethylenically unsaturated monomer, the process comprising steps (i), (ii) and (iii): (i) emulsion polymerization of monomers a11) and a12) to obtain a graft base a1), (ii) partial agglomerization of the graft base a1), and (iii) emulsion polymerization of graft monomers a21), a22) and optionally a23) onto the graft base obtained in step (i) or (ii), (i) emulsion polymerization of monomers a11) and a12) to obtain a graft base a1), (ii) partial agglomerization of the graft base a1), and (iii) emulsion polymerization of graft monomers a21), a22) and optionally a23) onto the graft base obtained in step (i) or (ii),