Hydrothermal processing is an important thermochemical conversion process that is used to convert biomass
into valuable products or biofuel. The process is usually performed in water at 250–37 4 1C under pressures of
4–22 MPa. The biomass is degraded into small components in wa ter. Based on the target products, i.e., bio-oil,
bio-gas or bio-carbon, the process conditions (temperature, pressure and time) are chosen. There has been
significant effort in evaluating various biomass resources for hydrothermal processing because the process is
suitable for any type of biomass including the co-utilization of biomass with waste materials. Additionally ,
because most biomass resources have high moisture contents, the most efficient way to process them is
through hydrothermal processing. To understand hydrothermal biomass processing and the degradation
pathway of biomass, it is necessary to understand the properties of water under hydrothermal conditions (i.e.,
subcritical and supercritical). In this respect, the physicochemical properties of water under subcritical and
supercritical conditions and the interactions of water with biomass are discussed in the present paper.