Various kinetic models for thermally initiated polymerization of styrene are compared concerning their description of isothermal batch and continuous polymerization as well as their prediction for runaway reactions. All models show good agreement for conversion in both isothermal and adiabatic situations, whereas predictions for molecular weight differ considerably. Pressure predictions according to Flory-Huggins and perturbed-chain statistical associating fluid theory equation of state (PC-SAFT EOS) also show good agreement considering a system from styrene/polystyrene only. For heterogeneous systems, such as suspension polymerization of styrene, various hazardous situations, like failure of the cooling system or cooling system and stirrer, are assumed. The pressure predictions for runaway reactions mainly depend on the assumptions for the phase behavior. Realistic predictions, which take into account the solubility of water in styrene/polystyrene, are only possible with PC-SAFT EOS.