The sodium salt of bis(2,2,3,3,4,4,5,5-octafluoro-1-pentyl)-2-sulfosuccinate (di-HCF4) was used as a surfactant to stabilize water-in-CO2 microemulsions under a range of conditions. The phase behavior of the surfactant/water/CO2 system was explored using cloud-point measurements for [di-HCF4] = 0.05-0.2 M, temperatures up to 55 degreesC, and pressures up to 35 bar, yielding the highest water and surfactant concentration ever achieved for water-in-CO2 microemulsion systems at comparable pressures. Stable microemulsions at [di-HCF4] = 0.1 M were studied by small-angle X-ray scattering over the range of water-to-surfactant molar ratio from 0.5 to 20. The micelles were observed to swell as water was added to the core.