Thermogravimetry, in situ electron spin resonance (ESR) spectroscopy and in situ and ex situ X-ray diffraction (XRD) are employed to investigate the thermal and catalytic degradation of a sample of commingled plastics (CP). XRD studies show CP to contain about 90% polyethylene (PE) and 10% polypropylene (PPE) and a smaller amount of TiO2. Analysis of the weight loss data in argon to 550 degrees C yields an activation energy E approximate to 38 kcal mol(-1) for the thermal decomposition of CP. In XRD studies, a melting point of 135 degrees C is inferred whereas the onset of irreversible degradation begins only around 360 degrees C. The in situ ESR experiments of CP and CP loaded with 10% Al2O3 (nanoscale) and 10% sulfur, both under 500 psig of H-2, show that for CP alone an ESR signal indicative of degradation is first seen near 380 degrees C, whereas for the loaded CP this degradation temperature is reduced to 280 degrees C. This enhanced catalytic degradation detected by ESR is believed to be due to elemental sulfur.