Combination of reverse micellar solutions and supercritical or compressed carbon dioxide (CO2) is a new and interesting topic. This work conducted the first study on the effect of compressed CO2 on the micro-properties (e.g., micropolarity, ionic strength, pH) of the sodium bis(2-ethylhexyl) sulfosuccinate reverse micelles in isooctane by phase behavior measurement, Fourier transform infrared and UV-vis spectroscopic techniques. The results show that CO2 can dissolve in both the organic-continuous phase and the water cores of the reverse micelles. The properties of the reverse micelles can be tuned continuously by changing the pressure of CO2 because the solubility of CO2 in the solution and in the water cores depends on the pressure. CO2 in the water cores can be ionized to produce HCO3-1 and H+. The micropolarity and ionic strength of the water cores increase with the pressure of CO2. Accordingly, the pH is reduced as the pressure and water-to-surfactant molar ratio (w(0)) are increased. (C) 2003 American Institute of Physics.