The effect of reaction time on the reactivity, conversion, and product distribution of coprocessing reactions in systems containing LDPE (low-density polyethylene), coal, and heavy petroleum resid was evaluated by performing reactions at 30-360 min. Individual reactions of LDPE and binary combinations of LDPE and coal were also performed. LDPE reactions with reaction times that increased from 60 to 360 min resulted in conversions that increased from 39.5% at 60 min to 90.2% at 300 min. After 360 min the conversion decreased to 70.9%. Similar results were obtained with the LDPE and coal reactions; increased reaction time resulted in increased LDPE conversion as well as increased overall conversion. In both reaction systems, lighter reaction products and more gases were produced as reaction time increased. The conversions and hexane solubles produced for ternary systems, containing either Maya or Manji resid, also increased with increased reaction time. The reaction solids produced in these systems at different reaction times were analyzed by Fourier transform infrared spectroscopy and differential scanning calorimetry. Increased crystallinity that directly correlated with longer reaction times was observed in reaction solids from the LDPE reactions. The reaction solids from the LDPE/coal and LDPE/coal/resid systems also increased in crystallinity after reaction, but the crystallinity was not correlated directly with reaction time. The combination of higher catalyst loading and increased retention time to 90 and 120 min resulted in higher conversions as well as the production of lighter products.