Three different potassium phosphates (KH2PO4, K2HPO4, and K3PO4) were tested as potential homogeneous catalysts for the hydrothermal liquefaction of soybean oil, soy protein, potato starch, microcrystalline cellulose, and their mixture. Na2CO3 and KOH, which have been widely applied in hydrothermal liquefaction (HTL) reactions, were also investigated for comparison to phosphates. The addition of K2HPO4 and K3PO4, which form basic solutions, led to yields of biocrude from HTL of starch and cellulose that were sometimes 2-5 times higher than those without phosphates. Adding Na2CO3, which also forms a basic solution, for HTL of polysaccharides also generated higher biocrude yields but not as high as those obtained with added phosphates. The use of KOH (another base) resulted in the highest yield of biocrude from HTL of the mixture. These additives, along with Na2CO3, also resulted in less solid residue being produced. The additives had almost no positive effect, however, on biocrude production from HTL of soybean oil or soy protein. The biocrudes produced from polysaccharides with added K2HPO4, K3PO4, and Na2CO3 have larger heating values and greater energy recoveries. The biocrudes mainly consist of acids/esters, alcohols, phenols, and ketones/aldehydes. The addition of phosphates led to a higher proportion of ketones/aldehydes at the expense of alcohols.