IR spectroscopy was applied to the study of micelle formation in aerosol OT [bis(2-ethylhexyl)sodium sulfosuccinate] systems. Dependence of the infrared spectra on surfactant concentration at fixed [H2O]/[surfactant] ratios in the range 0.2-10 has been determined at very low surfactant concentrations {1 X 10(-5) to 1.4 x 10(-3) mol.L-1} where microaggregate formation is observed. Effects on the molar extinction coefficients of the bands due to C-H stretching vibrations can be used as indicators of micelle formation, and they appear to be independent of the amount of water in the system. The results are discussed in terms of pseudophase and multiple equilibrium models of micelle formation.