Abstract Two major organic reactions in water under high temperature and high pressure, hydrothermal oxidation and gasification, possess great potential towards the thermochemical treatment of high moisture content organic wastes of different varieties and sources. Essentially, hydrothermal oxidation converts organic liquid and/or solid wastes to mainly carbon dioxide and water. Environmentally important and refractory organic pollutants such as polycyclic aromatic compounds, e.g. pyrene, naphthalene, phenanthrene, fluorene and biphenyl, and di-n-butyl phthalate, have been decomposed under oxidative hydrothermal conditions, with conversions of up to 99·9% obtained. Real world environmental wastes and pollutants have also been oxidised. The hydrothermal gasification process offers a new route for 'green' fuel production from biomass and biowastes resulting in a CO2 neutral energy technology. Generally, the main products of hydrothermal gasification are carbon dioxide, carbon monoxide, hydrogen, C1–C4 hydro...