Steady-state modeling and simulation of thermally initiated bulk polymerization of styrene in a train of existing industrial reactors is presented. Two reactors of different geometry and flow arrangement, which operate in series, are modeled as plug flow reactors. The model of Arai et al. (8) for the kinetics of thermal polymerization of polystyrene is used. The model predictions agree well with plant data for different operating conditions. A parametric study shows the feed rate, diluent (ethyl benzene) concentration and reactor temperatures to have a significant effect on final conversion and molecular weight.