294The design of any industrial-scale separation process requires some degree of knowledge about the thermophysical and chemical properties of the materials to be separated. This knowledge can take the form of experimentally measured data, particular for that system, or predictions obtained from a suitable mathematical model such as an equation of state (EOS) or empirical data correlation. It is generally considered better, from the process designer’s point of view, to use predicted properties, since calculations are far more economical to perform than are experimental studies. Unfortunately, there are relatively few reliable predictive methods currently available that have sufficient accuracy to be used in the design of supercritical fluid extraction (SEE) processes. This is because such processes usually involve high pressures and large, often polar, solutes. In addition, SEE processes often require operation near solvent critical points, where even good models of well-studied systems become marginal. This lack of data is slowly improving as more experimental and applied theoretical studies are completed. In this chapter, the more important thermophysical properties needed for supercritical fluid extraction design are discussed from the experimental point of view. Both the solvent and solute are discussed in terms of equilibrium and transport properties.