Reverse micelle and water-in-oil (w/o) microemulsion phases can be formed in near-critical and supercritical fluids, giving rise to uniquely pressure dependent phase behavior. The solvating power of reverse micelles formed from the surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in fluids with moderate critical temperatures (e.g., ethane, propane, or xenon) depends largely upon the water-to-surfactant ratio of the micelle phase (Wm), which at large Wm can approach that of bulk water. The maximum water-to-surfactant ratio (W0), which defines the boundary between a one-phase and a two-phase fluid system (where a second, predominantly aqueous phase exists), is strongly pressure dependent. The physical size of a reverse micelle in one-phase AOT/H2O systems at constant Wm has been shown to be nearly independent of the continuous-phase identity and pressure. In contrast, the apparent hydrodynamic size increases dramatically as W0 is approached due to increased micelle-micelle attractive interactions (e.g., clustering). The maximum reverse micelle size (Wm ∝ diameter) increases with pressure for fluids such as ethane and propane, approaching Wm = 40, corresponding to a droplet size of ∼17 nm. Significant micelle densities are obtained for two phase systems, even at relatively low pressure (<100 bar). These systems can be used to efficiently extract hydrophilic substances, including proteins, from dilute aqueous solution with substantial selectivity without the need for any chemical change to the system. © 1990 American Chemical Society.