We produced oils via hydrothermal liquefaction (HTL) of binary mixtures of biomass components (e.g., lignin, cellulose, starch) with different plastics and binary mixtures of plastics themselves. Cellulose, starch, and lignin demonstrated synergistic interactions (i.e., enhanced oil yields) with the plastics tested (polypropylene, polycarbonate, polystyrene, and polyethylene terephthalate). Polystyrene exhibited synergy during HTL with the three other plastics as did polypropylene during HTL with PET or PC. We used the experimental results to develop the first component-additivity model that predicts the oil yields from HTL of biomass-plastic and plastic-plastic mixtures. The model accounts for interactions among and between biomass components and plastic components in sub-, near-, and supercritical water. The model predicts 88% of 48 published oil yields from HTL experiments with mixtures containing plastics to within 10 wt%.