Water heated above 374°C and pressurized beyond 22.1 MPa possesses unique physical and chemical properties and is known as supercritical water. At supercritical conditions, water behaves like a nonpolar solvent and thus solvates organic molecules. Supercritical water has some of the desired properties of its liquid phase as well as the gaseous phase. Above the critical point, there is a drop in water properties like dielectric constant, viscosity, and density which enhance reaction kinetics along with the rise in diffusivity and free radical reactions. Supercritical water was first applied in hydrothermal conversion of organic compounds to gases in the 1970s (Amin, Reid, & Modell, 1975). The properties of water at various conditions are shown in Table 5.1. These changes in the property of water above critical point move the reaction toward gas formation rather than solid by-products as described latter in this chapter. Supercritical water gasification (SCWG) is one of the emerging green technologies for conversion of biomass to gaseous biofuels. Since water is used as reactant and reaction media in the SCWG process, wet biomasses or organic wastes are the most suitable feedstock. It helps in avoiding cost intensive drying of biomass which is desired in conventional thermochemical gasification of biomass. It mainly produces H2, CO2, CH4, and C2H6 at optimum conditions. Although the technology is promising, lower product yield is still a challenge. Higher product yield and lower operating cost can be achieved through reaction condition optimization and identifying an appropriate catalyst. Properties of Water at Various Conditions https://www.niso.org/standards/z39-96/ns/oasis-exchange/table"> Normal Water Subcritical Water Supercritical Water Temperature (°C) 25 250 350 400 400 Pressure (MPa) 0.1 5 25 25 50 Density, ρ (g/cm 3 ) 1 0.8 0.6 0.17 0.58 Dielectric constant, ε (F/m) 78.5 27.1 14.07 5.9 10.5 Ionic product, pKw 14 11.2 12 19.4 11.9 Heat capacity, Cp (KJ/Kg/K) 4.22 4.86 10.1 13 6.8 Dynamic viscosity, η (mPa s) 0.89 0.11 0.064 0.03 0.07 Source: Krammer & Vogel (2000), Kruse & Dinjus (2007).