N-octyl-quinolinium betaine is introduced as an absorption probe for the micropolarity of the interior of reverse micelles. Its solubilization by reverse micelles and water/oil microemulsions of Aerosol-OT in isooctane is compared with that of N-methyl-quinolinium betaine at various water contents of the solution. Analysis of the excitation energies in the visible range of the spectrum indicates that the methyl derivative probes the polarity of the aqueous pool of the micelle, whereas the octyl derivative behaves as a cosurfactant probe that reports on the polarity of the water/oil interfacial region. (C) 1994 Academic Press, Inc.