A review (including Author's recent works) covering the hydrodynamic behavior (residence time distribution, hydrodynamic dispersion) of sub or supercritical water in a tubular flow reactor, the kinetics of supercritical oxidation reactions, heat transfer in an autothermal tubular reactor containing sub or supercritical water, and illustrative oxidations and thermohydrolyses of C-H-O-N compounds including 2,4-and 2,6-diaminotoluenes, 2,2-diamino-4,4'-stilbenedisulfonic, acid, 2,2-dinitro-4,4 -stilbenedisulfonic acid, and p-nitrotoluenesulfonic acid.