Thermal polymerization of styrene in a lab-scale CSTR equipped with a pitched blade turbine impeller was simulated using a computational fluid dynamics (CFD) software package. The rotation of the reactor impeller was modeled using the multiple reference frames (MRF) technique. The path lines of the particles, released at the reactor inlet, were also generated to analyze the reaction progress throughout the reactor vessel The effects of the impeller speed, the input-output stream locations and the residence time were investigated The simulation showed the formation of a well-mixed region around the impeller and stagnant or slow moving fluids elsewhere in the reactor due to high viscosity of the polymer mass. The monomer conversion computed using the CFD model was in good agreement with that obtained from the CSTR model at low residence time. The input-output locations have a significant impact on the monomer conversion and the system homogeneity in the CSTR. (C) 2009 Elsevier Lid All rights reserved.