High density polyethylene (HDPE) and commingled post-consumer plastic waste (CP#2) were thermally and catalytically depolymerized in both a 27-cm(3) tubing reactor and a 150-cm(3) autoclave reactor at 400-435 degrees C, 60 min, under N-2 or H-2, with or without TiCl3 or HZSM-5 as catalyst. For both HDPE and CP#2, no significant non-catalytic reactions occurred until the reaction temperature reached 430 degrees C. For depolymerization reactions of HDPE and CP#2 in the autoclave reactor at 435 degrees C, the optimum oil yields (88.7% for HDPE and 86.6% for CP#2) were obtained over TiCl3 catalyst, while the maximum gas yields (21.2% for HDPE and 17.6% for CP#2) were produced over HZSM-5 catalyst. HZSM-5 catalyzed cyclization and aromatization reactions while TiCl3 catalyzed recombination and disproportionation reactions. Detailed product analyses and characterization lead to a reasonable explanation of reaction pathways and mechanisms.