The chemical modification of poly(ethylene-co-vinyl acetate) with a primary aliphatic n-alcohol in a modular co-rotating twin screw extruder was investigated The purpose was to build a theoretical model, able to predict the extent of conversion along the screw axis. First, the influence of operating conditions (throughput, screw speed, barrel temperature and initial reactant concentrations) on the EVA copolymer conversion was experimentally examined. Then, the extent of conversion was computed by coupling flow simulation and reaction kinetics. Theoretical and experimental results were found in good agreement, what allows the prediction of the reaction extent as a function of the processing conditions.