In this work, several applications are reviewed and the importance of phase equilibria is discussed. In the reaction of cellulose in subcritical and supercritical water for producing chemicals, cellulose decomposes to form lower carbon number sugars, oligomers, and various acids. As conditions change from a subcritical reaction environment to a supercritical reaction environment, we observed a drastic change of the kinetic rate constant, which is thought to be the result of the cellulose-water system changing from a heterogeneous state to a homogeneous state. Diamond anvil cell measurements (DAC) are used to provide Visualization of phase changes in the cellulose-water system. In the reaction of polyethylene terephthalate (PET) in supercritical water for recycling, PET decomposes to form oligomers, ethylene glycol, and the monomer, terephthalic acid. The reaction kinetics for this polymer-water system is much slower and requires 10 min for 100% conversion vs. 50 ms for 100% conversion of cellulose. The reason is thought to be due to the number and type of reaction phases present in the system, that is, solid-fluid or liquid-fluid phases. In the hydrogenation of heavy oils for energy conversion applications, the reaction kinetics can be very slow. Supercritical water provides a homogeneous reaction environment for the hydrogen of heavy oil system. However, for this case, we have found that an excellent hydrogenating atmosphere can be made by addition of oxygen, which apparently provides active hydrogen from CO formed, through the water-gas shift reaction. In catalytic reactions of heavy oils, in-situ extraction of coke precursors is provided by the supercritical water environment. Further studies are reported on the oxidation of hydrocarbons. In the treatment of high-level liquid wastes from the nuclear industries, it is found that many metals can be removed from solution by raising the temperature and pressure of the aqueous solutions. These results are discussed in view of recent supercritical water metal oxide solubility and supercritical water density measurements.