The thermal oxidation of polypropylene was probed, in situ, by a novel technique that simultaneously monitors chemically induced light emission (chemiluminescence) and changes in FTIR emission spectra of an oxidizing sample. The results obtained from this technique demonstrated that the chemiluminescence intensity (I-CL) was proportional to the accumulation of carbonyl species formed during oxidation, rather than the rate of oxidation, which is predicted from application of the steady-state approximation to classical chemiluminescence mechanisms associated with auto-oxidation. Two possible mechanisms were proposed to account for the results obtained. The first mechanism involved energy transfer from a triplet carbonyl, formed via a classical mechanism, to a more efficient emitter formed during oxidation. The second possible mechanism involved reaction of a peroxidic compound with an oxidation product.