The solid superacid-catalyzed depolymerization-liquefaction (DL) reactions of high-density polyethylene (HDPE), isotactic polypropylene (PPR), and cis-polybutadiene (PB) samples were systematically investigated as a function of processing conditions, i.e., temperature (350-450 degrees C), time (0.5-3.0 h), H-2 pressure (500-2000 psig), catalyst type and concentration, and the presence of solvents. Catalysts used included SO42-/Fe2O3, SO42-/ZrO2, and a Pt-modified SO42-/ZrO2. At temperatures >400 degrees C, with 1-2 wt % of SO42-/Fe2O3 or SO42-/ZrO2 as catalyst, there is an overlap of catalytic and noncatalytic, viz. thermal DL, reactions. Under such conditions HDPE yields a Liquid product consisting of C-5-C-30 (mostly C-5-C-12) normal and branched paraffins, accompanied by small amounts of cycloparaffins and olefins. Selective catalytic DL of HDPE was achieved at lower temperature (350 degrees C) in the presence of 17-33 wt % of SO42-/ZrO2 or the more active Pt/SO42-/ZrO2 catalyst, preferably in the presence of a chemically compatible solvent, i.e., n-octadecane. Under such conditions the high-yield (>90 wt %) product predominantly consists of branched paraffins in the gasoline boiling range. PPR, which shows high DL reactivity due to its multiply branched polymeric chain structure, yields a similar gasoline-like mixture of C-5-C-12 branched paraffins as main product. The change in product composition from HDPE and PPR as a function of temperature and reaction time allows for elucidation of mechanistic aspects of the stepwise DL reactions of these polymers). For HDPF results are rationalized in terms of a carbonium ion mechanism involving extensive skeletal isomerization and attendant beta-cleavage reactions leading to low branched paraffins as final products. Results obtained demonstrate that at mild temperatures, under properly designed catalytic conditions, waste HDPE and PRR feeds could be effectively converted into desirable multibranched paraffins which represent potential blending components for reformulated, nonaromatic gasolines.