Biomass is a promising renewable and sustainable resource to produce energy and value-added chemicals. Fast pyrolysis is one of the simplest thermochemical methods to convert biomass into high yields of liquid products that can be upgraded into drop-in fuels or platform chemicals; however, its diverse product distributions and low product selectivity incur significant cost due to subsequent upgrading and separation operations. In this work, a strategy to promote yields of low-molecular-weight products (LMWPs) from cellulose pyrolysis via hydroxyl group stabilization using molten polymers (MPs) is presented. Three types of thermoplastics, high-density polyethylene (HDPE), polyethylene glycol (PEG), and polystyrene (PS), were copyrolyzed with cellulose to investigate the possible hydroxyl group stabilization effects caused by the ether and aromatic moieties in MPs during cellulose pyrolysis. A custom-made batch pyrolysis reactor was employed for the copyrolysis experiments. Our results showed that the combined yields of levoglucosan (LG) and LMWPs significantly increased in the presence of MPs due to the physical inhibition of anhydrosugar oligomer evaporation. The product distributions were varied dependent on the MPs used. In particular, both ether groups in PEG and aromatic groups in PS were found to stabilize the cellulosic hydroxyl groups during glycosidic bond cleavage, inhibiting the formation of LG. Aromatic moieties in MPs were observed to create a stronger inhibition effect on the glycosidic bond cleavage than ether moieties. Our experiments also suggest that both ether and aromatic groups in MPs stabilize the hydroxyl groups during dehydration, leading to increased yields of products from retro-Diels Alder fragmentation. Ether moieties were found to be more effective at inhibiting dehydration than aromatic moieties. Yields of the HDPE-derived products increased during copyrolysis, suggesting a possible catalytic effect in HDPE pyrolysis caused by the LMWPs produced from carbohydrate pyrolysis. Yields of the PEG-derived products increased only in the presence of cellulose and glucose, whereas yields of the PS-derived products were unaffected in all copyrolysis experiments. A possible reaction mechanism accounting for the hydroxyl group stabilization effects is proposed based on our experimental findings.