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Oxidation, pyrolysis and hydrolysis of methane, ethane, methanol and to some extent heptane and toluene, in supercritical water or nitrogen, at temperatures to 600°C and pressures to 1000 bar were studied. Temperatures of the reactive mixtures were always above the respective critical curves. Oxidation was performed with oxygen or air injection, flameless or with diffusion flames. High pressure, non‐corrosive reaction vessels of different types and the complementary apparatus for high pressure injection and analysis are described. Flameless oxidation of methane in homogeneous supercritical aqueous mixtures produces CO, CO2, H2 and CH3OH. Methanol formation can reach 30% of reacted methane, “selectivity”, at 600 bar. High pressure increases the methanol selectivity, flame reaction decreases it. Overall rate constants are given. Similar studies were made with supercritical aqueous ethane mixtures. Hydrothermal pyrolysis, “hydrothermolysis”, was investigated. Even at 600°C and 600 bar less than 3% methane decomposed after 120 minutes. Pyrolysis of methanol at 500°C and 600 bar is much slower in an aqueous environment than in nitrogen. Hydrothermal soot formation was studied with several hydrocarbons in supercritical fluids. Water, as compared with argon, as a mixture component appears to suppress soot formation. CO conversion with supercritical water was also quantitatively pursued.