An experimental study of the peroxide-induced degradation of polypropylene in a corotating twin screw extruder, operating under various conditions, was reported. Runs were performed without and with peroxide in different concentrations. The evolution of the chemical reactions along the extruder was characterized by on-line rheological measurements and by determination of the molecular weight of samples collected at the same locations. The results evidenced the relationships between peroxide concentration and processing conditions with rheological properties and molecular structure of the modified materials. The concept of chain-scission distribution function elucidated the mechanisms involved in the thermomechanical degradation of PP, which is by chain scission and is dependent on the level of stress imposed by the screw elements, temperature, and concentration of hydrogen-abstracting agents. (C) 2003 Wiley Periodicals, Inc.