This article describes the copolymerization of styrene-butylmethacrylate (St-BMA) and the homopolymerization of styrene (St) in a counterrotating twin screw extruder. The effect of prepolymerization on both the product properties and process was studied. It turned out that the process of reactive extrusion was strongly influenced by prepolymerization. Also, the product properties were altered by prepolymerization. When the polymerization of St is compared with the copolymerization of St-BMA, the process of St was more stable, although the polymerization rate was lower.