During the reactive processing of HD Polyethylene molecules are known to form branched structures, eventually resulting in a MFI decrease. This phenomenon was studied by experiments in a twin-screw extruder and measurement of the MWD by SEC-MALS. A Monte Carlo simulation model has been setup to calculate the MWD and branching distribution. The model includes both branching and random scission. Branch points are assumed to be created by coupling of vinyl ends of linear PE to secondary radical sites on PE backbones, yielding tertiary radical sites that undergo termination by disproportionation. This mechanism was recently proposed by Camara et al. [1]. Comparing the computed MWD to SEC-MALS data allowed determining the kinetic coefficients of the branching and scission reactions. Random scission turns out to be weak but still significant to prevent the formation of an extended MWD tail. For the found parameters perfect agreement was found between measured and computed MWD for various initial peroxide concentrations. These findings contribute to a better fundamental understanding of the simultaneous branching and scission during PE modification. It forms a firm support of the vinyl end to secondary radical coupling branching mechanism. Furthermore, it is shown that the MC simulations provide explicit branching topologies as a basis for predicting, for instance, complex rheological behavior of the modified polymer melt (Read et al. [2]). (C) 2013 Elsevier Ltd. All rights reserved.