For the industrial expandable polystyrene (EPS) process, the Predici software package is successfully applied to demonstrate that the composite k(t) model is the most appropriate one to accurately account for diffusional limitations on termination. For a broad range of conditions, the reported set of model parameters allows an excellent description of experimental data on monomer conversion and molar mass distribution (MMD). For low temperatures and dicumylperoxide amounts (<403 K;<0.20 m % DCP), a bimodal log-MMD is obtained, which can be explained by the inability of chain transfer to attenuate the gel-effect. For the opposite conditions, a unimodal log-MMD results as the composite k(t) model provides an excellent description of the termination rates of the by chain transfer formed radicals. A unimodal log-MMD also follows by adding a sufficiently high amount of the blowing agent n-pentane (approximate to 20 m %). The impact of degradation reactions on the EPS product can be neglected. (c) 2016 American Institute of Chemical Engineers AIChE J, 63: 2043-2059, 2017