This paper reports data obtained from monitoring the evolution of chemical conversion and morphology during the production of a compatibilized polymer blend of PA 6/EPM/EPM-g-MA and of modified polyolefins (grafted with MA) in a co-rotating twin screw extruder In the first case, the MA content of the modified rubber decreases strongly to almost zero-level and the particle size reduces dramatically from mm to sub-mu m level already in the melting zone. The process seems to be mainly influenced by the local flow characteristics, as they assure efficient generation and renewal of interfacial area. In the second case of MA grafting, a similar evolution is observed regardless of the polyolefin type but, for this kind of reactions, temperature and residence time play an important role. A brief presentation of the on-line monitoring tools used in the above studies is also included.