There has been relatively little effort to quantitatively understand graft copolymerization in either batch mixers or twin-screw extruders. Most efforts have concentrated on grafting maleic anhydride, which does not homopolymerize. In this paper we consider grafting with styrene, which may homopolymerize as well as graft. The influence of residence time on degree of grafting in an internal mixer and a twin-screw extruder were studied by measuring reaction yields with respect to reaction time in a mixer and along the screw axis in a twin-screw extruder. The degree of grafting increased with initial monomer and peroxide concentration. Grafting reactions with three different peroxides were also investigated. The degree of styrene grafting in an internal mixer is slightly higher than that in a twin-screw extruder. The rate of reaction along the screw axis in terms of residence time seems higher than for the batch mixer. The melt viscosity dropped dramatically with addition of peroxide. A kinetic scheme is proposed and the experimental results are critically compared with it.