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Optical emission spectroscopy was used to monitor the evolution of chemical reactions taking place on the surface of semicrystalline polypropylene during oxygen plasma treatment. The optical spectra were continuously acquired during treatment at different pressures (20-140 Pa). The time evolution of different excited radicals (O, H, CO, OH, and CH) allowed for the estimation of the chemical reactions. The oxidation of the material started as soon as the plasma was ignited and slowly intensified until the evaporation of the material was indicated by the appearance of the CH band. As the CH band appeared, a rapid increase in CO and OH was observed, together with a drop in O and a maximum in H. A well-defined minimum in the time needed for the appearance of the CH band was found versus the pressure. The results were explained by heating the polymer due to exothermic physical and chemical reactions and cooling by the surrounding gas.