The long-term viability of a supercritical water-cooled reactor (SCWR) will depend on the ability of designers and operators to control and maintain water chemistry conditions that will minimize corrosion and the transport of both corrosion products and radionuclides, at a pressure of 25 MPa and temperatures from 300°C to 625°C. To achieve this goal, the behavior of low concentrations of impurities such as transition metal corrosion products, chemistry control agents, impurities in the feedwater, and radionuclides (fission and activation products) in subcritical and supercritical water must be understood. A second key aspect of SCWR water chemistry control will be mitigation of the effects of water radiolysis. Preliminary studies suggest markedly different behavior than that predicted by extrapolating conventional water-cooled reactor behavior. The principal challenge in predicting corrosion and fission product transport is the lack of thermochemical and kinetic data above 300°C. Calculations with extrapolated data show that the formation of neutral complexes increases with temperature and can become important under near-critical and supercritical conditions. The most important region is from 300°C to 450°C, where the properties of water change dramatically and solvent compressibility effects exert a huge influence on solvation. The potential for increased transport and deposition of corrosion products (radioactive and inactive), leading to increased deposition on fuel cladding surfaces and increased out-of-core radiation fields and worker dose, must be assessed. The commonly used strategy of adding excess hydrogen at concentrations sufficient to suppress the net radiolytic production of primary oxidizing species may not be effective in an SCWR. Because direct measurement of the chemistry under such extreme conditions of temperature, pressure, and radiation fields is difficult, the most promising approach involves a combination of theoretical calculations, chemical models, and experimental work.