Aggregate structures in water-in-CO2 microemulsions mere studied by high-pressure small-angle neutron scattering(SANS). With liquid CO2 at 15 degrees C, the partially fluorinated, di-chain surfactant bis(1H,1H,5H-octafluoro-n-pentyl) sodium sulfosuccinate (di-HCF4) stabilized single-phase microemulsions at pressures above similar to 400 bar. The maximum water loading (w) investigated was 30 ([water]/[di-HCF4) representing formation of relatively large water droplets in the microemulsion. Between w = 5 and 30, the SANS data were consistent with a model for attractive polydisperse spherical droplets. A linear relationship between the water droplet radius (R-c) and w was found, which gave an apparent head group area for the surfactant of 87 Angstrom(2) at the water-CO2 interface. In Winsor II type microemulsions the value of R-c, measured in the presence of excess water; increased with pressure from 36 Angstrom at 400 bar to 56 Angstrom at 550 bar.