Low-pressure pyrolysis experiments employing a high molecular weight polymer and a model compound for coal were conducted to address the effect of phase behavior on the overall degradation mechanism and reaction pathways during coprocessing. Thermal degradation of high-density polyethylene (HDPE) and 4-(1-naphthylmethyl)bibenzyl (NBBM) was conducted at 420 degrees C at different reactant loadings, both neat and in binary mixtures. During binary mixture experiments, there was an enhancement in the selectivities to primary products of NBBM at longer reaction times, with a significant reduction in the formation of secondary and tertiary products. These favorable interactions occurred because the polymer induced diffusion limitations in the system compared to neat NBBM pyrolysis, which minimized NBBM self-interactions and promoted reactions with the surrounding polymer. The degradation pathways of HDPE during binary mixture experiments were similar to those during neat reactions, with slight changes to the product yields that were consistent with increasing the overall reactant loading during neat pyrolysis. Variation of the relative reactant loadings indicated that the favorable feedstock interactions were still realized with only slight alterations in the product slate for NBBM and increased yields of saturated species derived from HDPE. Overall, the experiments carried out demonstrated that favorable interactions exist in both the liquid and gas phases during coprocessing, and primary reaction pathways and the mechanism governing the interactions between the feedstocks were elucidated.