The very special physical properties of supercritical fluid (SCF) solutions have led to intense attempts to exploit them as solvents for separations and reactions. SCFs are characterized by high density, leading to very high loadings of solutes, low viscosity, and high molecular diffusivity. Moreover, they exhibit compressibilities typically one to three orders of magnitude greater than other fluids. Since solubilities appear to be virtually exponential in density, this means that very small pressure changes can result in enormous solubility variations, giving the design engineer the opportunity to pull chemicals into solution or drop them out very selectively. All of these properties make SCFs ideal candidates for mass transfer solvents.