A process for the preparation of expandable polystyrene (EPS) containing particles of carbon black and/or graphite is disclosed, comprising: (a) forming a masterbatch by adding carbon black and/or graphite to styrene polymer, (b) adding said masterbatch to an organic phase comprising styrene and optional comonomers, and heating the organic phase to a temperature between 30 and 90° C., (c) mixing the organic phase with an aqueous phase comprising water and suspending agents preheated to a temperature between 80 and 120° C. which is higher than the temperature of the organic phase, and (d) polymerizing in a suspension polymerization reaction the styrene and optional comonomers contained in the mixed aqueous and organic phases, with an expanding agent being added before, during or after the polymerization, wherein the styrene polymer in the masterbatch contains at least 1 000 ppm of dimers. The process gives an improved product.