The polypropylene (PP) was coated on porous cr-alumina particles and then pyrolyzed in a flow of helium or a mixture of helium-oxygen at atmospheric pressure. The mass release from PP was dramatically enhanced in the presence of oxygen at temperatures in the range of 200-300 degrees C, The temperature for the 50% mass release was ca. 250 degrees C at an oxygen partial pressure (P-o2) of 16.7 kPa and was lowered by 190 degrees C as compared with a system which contained no oxygen. When P-o2 was higher than 4.2 kPa, the mass release rate obeyed first-order kinetics with respect to P-o2, and the activation energy was calculated and found to be 65-75 kJ/mol. The activation energy was considerably lower than that for pyrolysis in the absence of oxygen (230 kJ/mol) and agreed well with the value for formation of peroxides on tertiary carbons. When the pyrolysis was conducted at 250 degrees C under P-o2 = 16.7 kPa, the carbon-based yield of volatiles exceeded 90%, and the yields of CS2-soluble oil and wax were 76% and 6.0%, respectively. The carbon-based yields of other products were: Acetone, 2.5%; methanol, 1.5%; CO, 3.0%; CO2, 2.2% and solid residue; 10%. Proton NMR (nuclear magnetic resonance) analysis showed that the CS2-soluble oil possessed an elemental composition of C100H187O11. The average number of carbon atoms per molecule of the oil was approximately 10, and -CH2-OH, -C(CH3)C=O and =CH2 were typical end groups. Their formation is explained by the decomposition of peroxides formed on tertiary carbons.