Heterogeneous catalysts have so far mostly been used in gasification processes, where they are reported to have a significant positive effect on low-temperature processes. In addition, during hydrothermal liquefaction some gasification is crucial, since oxygen is removed dur­ing this process. However, extensive gasification will reduce the bio-oil yield. Nickel, palla­dium, and platinum catalysts were tested during gasification of cellulose at 350°C, 25 MPa, and 10-180 min reaction time, and it was reported that mainly methane and carbon dioxide were produced over supported nickel catalysts, whereas mainly hydrogen and carbon diox­ide were produced over supported palladium and platinum catalysts. Most likely the gas is produced by direct gasification of aqueous compounds of the primary biomass degradation (Minowa and Inoue, 1999). Various other heterogeneous catalysts have been tested in hydro­thermal conversion processes; however, the main focus has been to improve gasification, not liquid yields. Examples of these catalysts are Ni/Al2O3, Ru/TiO2, and ZrO2 (Elliot et al., 1993; Elliot et al., 1994). Catalysis of gasification at conditions below 400°C was extensively reviewed by Peterson et al. (Peterson et al., 2008). In a rare study of heterogeneous catalysts at semihydrothermal conditions, Watanabe et al. (Watanabe et al., 2006) tested the effect of zirconia (ZrO2) on stearic acid (C17H35COOH) decomposition at 400°C and 25 MPa for 30 min. Zirconia has a high density of amphoteric sites on the surface, which means that it potentially promotes both acid and base-catalyzed reactions. They observed that zirconia (ZrO2) enhanced the conversion of the C17-acid, and the main products were the C16-alkene, acetic acid, and 2-Nonadecanone.