A model reactive system was defined for studying experimentally and by simulation the competition between reaction and diffusion of two low molecular weight reactants, 2,3-epoxypropyl-phenylether (EPPE) and dipentylamine (DPA). Both reactants are miscible in a high-viscous molten polymer, poly(ethylene-co-vinyl acetate) (EVA). The comparison of the experimental rates of reaction for initially homogeneous samples and bi-layer unpremixed samples proved that the reaction was diffusion controlled. A kinetic model of the epoxy-amine reaction was coupled to mutual diffusion coefficients of reacting species in a transport model and the simulations were compared with experimental results. The diffusion/reaction process was finally related to typical mixing conditions encountered in reactive polymer processes. For the model reactive system, the simulations have established that actual mixing conditions with shear rate values encountered in polymer processing machines, were able to homogenize the system in less than 10 s. In other words, the reaction should no longer be controlled by molecular diffusion as soon as a relatively low intensity mixing is applied (shear rate > 10 s(-1)). (c) 2007 Elsevier Ltd. All rights reserved.