In situ electron spin resonance (ESR) spectroscopy is employed to investigate the thermal and catalytic degradation of a sample of commingled plastics (CP) containing about 95% polyethylene and 5% polypropylene. The materials tested include elemental sulfur (S), NiMo/Al2O3, and zeolite MZSM-5 for temperatures between ambient and 450 degrees C and pressures up to 500 psig of H-2. The depolymerization temperature T-d of CP, where an ESR signal is first observed, is about 360 degrees C thermally. With 1 wt % loading of CP with S, T-d is reduced to about 250 degrees C. With 10 wt % S plus 10 wt % NiMo/Al2O3, T-d is further reduced to about 230 degrees C and the free radical intensity N is reduced considerably compared to the CP + S case. HZSM-5 does not affect the T-d of CP, but N is reduced considerably and with. increase in temperature, N decreases, similar to the NiMo/Al2O3 case. These results, along with the observed increase in the ESR, line width for the HZSM-5 and NiMo/Al2O3 cases, suggest capping of the free radicals by hydrogenation. For sulfur loading of CP, the significant increase in N and lowering of % (also observed by thermogravimetry) suggest enhanced depolymerization of CP. This sulfur-promoted depolymerization of polyethylene/ polypropylene is supported by the liquefaction experiments of Sivakumar et al, (Fuel Process. Technol, 1996, 49, 219) where sulfur loadings of polyethylene and polypropylene produced a nearly 2-fold increase in the gasoline-range oil yield.