Coprocessing of waste plastics with coal and with petroleum resid was investigated to determine the effect of resid on reactivity and conversion. The coal used in this,study was Blind Canyon bituminous coal, the resids were Maya and Manji, and the model plastics tested were polystyrene, poly(ethylene terephthalate) (PET), and low-density polyethylene (LDPE). Tl;ree systems, the individual species, binary combinations, and ternary combinations, were reacted at conditions of 430 degrees C and 8.7 MPa of H-2 introduced at ambient temperature for 60 min of reaction time. Presulfided NiMo/Al2O3 was used as the catalyst, typically at 1 wt % loading, although other catalyst loading levels of 3 and 10 wt % were tested. Under these conditions polystyrene and PET reacted readily, while LDPE was difficult to convert. Binary reactions with resids resulted in high conversions of similar to 94% from polystyrene and PET, while the reactions with LDPE yielded somewhat less conversion of similar to 72%. By contrast, reactions of plastics with coal converted substantially less, ranging from 70.2% for polystyrene and coal to 39.9% for LDPE and coal. Ternary reactions with coal, plastic, and resid resulted in high conversions for all systems (similar to 89-95%) except those with LDPE (similar to 77-81%). The effect of coprocessing binary and ternary systems compared to individual systems on the basis of conversion, hexane solubles, and gas productions was determined. The effect of adding a third species into the binary systems was also evaluated. The hexane-soluble products from the three reaction sets were analyzed by simulated distillation to determine the amount of the reaction product boiling at less than 500 degrees C. Reactions containing LDPE produced substantially less material that boiled below 500 degrees C than did the other reactions.