A mixture of CO2, water, methyl orange (MO) and Dynol 604 surfactant was analyzed at 309 K and 20 MPa simultaneously by impedance and UV-vis spectroscopy. A plot of the static dielectric constant as a function of the surfactant concentration was obtained using a suitable equivalent circuit to elaborate impedance data. Experimental points can be fitted by two different straight lines whose intersection occurs at about 0.13% (w/w) concentration of Dynol. When the surfactant concentration crossed the aforementioned value, a peak was detectable in the UV-vis spectrum at a wavelength range corresponding to the absorption of MO; absorbance of the peak increased with the surfactant concentration. As it has been already shown that Dynol 604 stabilizes microemulsions of water in CO2, these preliminary results suggest that measurements of dielectric constant can be used to detect micellization of amphiphiles soluble in supercritical CO2. (C) 2006 Elsevier B.V. All rights reserved.