Abstract: The present invention relates to a process for preparing ABS polyblends having an alkenyl aromaticalkenyl nitrile monomer matrix polymeric phase reinforced with rubber particles grafted with said monomers and having a bimodal rubber particle size distribution, the process comprising: charging continuously a formulation of said monomers having a soluble diene dissolved therein and a first grafted diene rubber particle dispersed therein to a back-mixed reaction zone operating at 50-80% steady state, flow-through, conversion of said monomer formulation, continuously mass polymerizing said charged monomer formulation to 50-80% conversion, forming and dispersing said soluble rubber as second grafted diene particles having a particle size larger than said first grafted rubber particles, continuously withdrawing an effluent from said reaction zone and separating said ABS polyblend from said effluent.