The mechanism of chemiluminescence (CL) during thermal oxidation of polypropylene (PP) was probed by doping PP with an energy acceptor (9,10-dibromoanthracene [DBA]) and a chemiluminescence (CL) activator (9,10-diphenylanthracene [DPA]). Doping PP with DBA had little effect on the shape of the CL intensity (I-CL)-time profile. This suggests that energy transfer from triplet states is probably not significant in the scheme of PP CL. However, the CL activator (DPA) had a significant effect on the shape and intensity of the I-CL-time profile. In the absence of DPA, the I-CL-time profile matches the profile for the formation of carbonyl-containing oxidation products from FTIR-emission spectra. In contrast, in the presence of DPA, it was the integrated DPA I-CL-time profile which snatched the oxidation product profile, indicating that now I-CL was proportional to the hydroperoxide concentration. It is suggested that peroxides formed during PP oxidation are capable of reacting with DPA to produce chemically induced electron exchange luminescence (CIEEL). It is also suggested that CL from undoped PP, i.e., direct CL, may also occur by a CIEEL mechanism. This mechanism is believed to involve the reaction of PP peroxides with an easily oxidizable luminescent oxidation product. For the detector system used in this study the majority of light emitted is consistent with reactions between acyl peroxides and alpha,beta -unsaturated carbonyls. The consequences of this mechanism are that the I-CL-time curve measured during the oxidation of PP may reflect either the hydroperoxide profile or the oxidation product profile depending on the spectral wavelength analyzed or the state of purity of the polymer.