Abstract Since the early 1990s, Adschiri and his colleagues have performed research on the use of supercritical water in diverse applications, including heavy oil reforming, waste polymer decomposition and chemical raw materials recovery, cellulose hydrolysis and sugar recovery, lignin decomposition and chemical raw materials recovery, and nanoparticle synthesis. Regarding inorganic materials synthesis, they invented supercritical hydrothermal synthesis for the continuous flow synthesis of metal oxide nanoparticles, which has already found worldwide industrial applications. They proposed a two-fluid mixing flow system where an aqueous metal salt solution is mixed with supercritical water to heat the solution to the supercritical state within a very short time, that is not attainable by conventional reactor systems. Organic-inorganic hybrid nanoparticles synthesis was demonstrated in the supercritical state, realizing homogeneous phase formation of organic molecules at high loadings. By optimizing the reaction conditions, small facet-controlled nanoparticles with narrow particle size distribution can be obtained. Organic modification of nanoparticles is shown to be effective for fabricating highly concentrated nanohybrid polymers or nano inks. Furthermore, these synthesized facet-controlled nanoparticles show high catalytic activities. Indeed, it is demonstrated that steam reforming of heavy oils or biomass wastes (black liquor) can occur in water even at low temperatures.