The temperature-programmed reaction (TPR) technique coupled with the in situ infrared (IR)/mass spectroscopy (MS) analysis has been employed to study the thermal decomposition of polypropylene. These techniques, especially when coupled with a gas chromatograph (GC), are effective for determining thermal stability, product distribution and product evolution as a function of temperature. The polymer decomposes at approximately 475 K yielding hydrocarbons in the C-1-C-7 range, with propylene, C-5 and C-6 composing the majority of gaseous products. Decomposition over alumina and 5 wt% Ni/Al2O3 with and without the presence of hydrogen does not affect thermal stability. Also, decomposition over alumina with and without the presence of hydrogen does not affect the depolymerization pathway. The popular beta-scission/backbiting mechanism cannot fully explain observations from this study. Possible alternatives are explored.