Concentrated CO2-in-water (C/W) emulsions are reported for amphiphiles containing alkylene oxide-, siloxane-, and fluorocarbon-based tails as a function of temperature and salinity. Poly(ethylene oxide)-b-poly(butylene oxide) (EO15-b-BO12) can emulsify up to 70% CO2 with droplet sizes from 2 to 4 mum in diameter, as determined by video-enhanced microscopy, This emulsion is stable over 48 h against both flocculation and coalescence. In contrast, it is extremely difficult to form concentrated water-in-CO2 (W/C) emulsions with surfactants containing alkylene oxide moieties due to limited solvation of such tails by CO2. In several cases, C/W emulsions are formed even when the surfactant prefers CO2. This violation of Bancroft's rule may be attributed in part to the low viscosity of the compressed CO2, which governs several mass and momentum transport mechanisms relevant to emulsion formation and stabilization. For the first time, WIC microemulsions are observed in a system with a nonionic amphiphile, namely F(CF2CF2)(3-8)CH2CH2O(CH2CH2O)(10-15)H. For the same system, the emulsion morphology changes from C/W to W/C as the temperature increases. The electrical conductivity of C/W emulsions is predicted successfully as a function of the dispersed phase volume fraction of CO2 with Maxwell's theory for inhomogeneous systems.