Synchrotron radiation small-angle X-ray scattering (SAXS) technique was used to quantitatively derive structural information of the Dynol-604 (a surfactant) based water-in-CO2 reverse micelles. By using the Guinier plot (In I(q) vs. q(2)) on the data sets in a defined small q range (0.025 -0.040 Angstrom(-1)), the radii of the reverse micelles at different pressures and loading water were estimated, which were in the range of 73.8 to similar to 78.1 Angstrom. (C) 2002 Elsevier Science B.V. All rights reserved.