Polystyrene (PS) was modified by reactive extrusion with trimethylolpropane triacrylate (TMPTA) and dicumyl peroxide (DCP). The coupling reaction caused by TMPTA increased the molecular weight of PS, and this coupling reaction was enhanced in the presence of DCP at high TMPTA/DCP ratio. The theological properties of the extrudate were examined. The impact strength of PS improved as the molecular weight increased by the coupling reaction. (C) 2004 Wiley Periodicals, Inc.