We report the properties of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) microemulsions formed in supercritical hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons, and fluorocarbons. The fluids used in this study include compounds that are of low toxicity and flammability and that are expected to remain environmentally acceptable well into the next century (e.g., 1,1,1,2-tetrafluoroethane (R134a) and chlorodifluoromethane (R22)). We show that it is possible to form a water-in-oil type of microemulsion in a low molecular weight HCFC (R22). In addition to these HCFCs, we also review the ability to form microemulsions in 14 other fluids (ethane, propene, propane, n-butane, n-pentane, n-hexane, isobutane, isooctane, difluoromethane, trifluoromethane, hexafluoroethane, sulfur hexafluoride, xenon, and carbon dioxide) at conditions just above or below the critical point (0.75 < T/T-c < 1.1) of the solvent. Due to the proximity of these liquids to the critical point, it is possible to make substantial changes in the densities of these solvents with modest changes in pressure. These 16 fluids have contrasting physical and chemical properties. We find that the parameter which universally predicts the ability of these solvents to form a microemulsion is the high-frequency dielectric constant (UV-vis light frequencies). This solvent dielectric constant is the parameter that governs the magnitude of the intermicellar van der Waals attractive forces but may also be relevant to the short-range attractive forces(surfactant tail to surfactant tail) that possibly control the phase behavior of these systems. We report extensively the phase behavior of AOT and didodecyldimethylammonium bromide microemulsions formed in a supercritical HCFC, R22. Microemulsions formed in supercritical R22 were demonstrated to have strongly density-dependent maximum molar water-to-surfactant ratios, W-0. When the pressure is increased from 100 to 400 bar, W-0 increases from 5 to 50, making the solvency of the polar or ionic species in these systems highly pressure tunable. It was also shown that HCFC-based microemulsions are capable of solubilizing high molecular weight proteins, such as cytochrome c, which demonstrates their usefulness for separations from aqueous solutions. We show that microemulsions in HCFCs are practical alternatives to other fluids, such as supercritical carbon dioxide.