The states of water and the conformations of sodium bis(2-ethylhexyl)sulfosuccinate (Aerosol-OT, AOT) in the reverse (water-in-oil, W/O) microemulsions system of AOT/isooctane/water have been investigated by using Fourier transform infrared spectroscopy (FT-IR) techniques. The broad peaks obtained for hydroxy of water and for carbonyl of AOT molecules have been resolved by least-squares curve fitting. Hydroxy stretching vibration bands of water are composed of four peaks located at 3610 +/- 20, 3540 +/- 20, 3440 +/- 20, and 3225 +/- 20 cm(-1), respectively. They are assigned to trapped water in the palisade layer, bound water with the sulfo group, free water, and bound water with sodium counterion in the microemulsion droplets. The asymmetric adsorbed peak of carbonyl stretching vibration of AOT molecule is composed of two peaks located at 1739 +/- 1 and 1725 +/- 2 cm(-1), which corresponds to different carbonyl in gauche conformation and trans conformation of AOT molecules, respectively. The structural parameters of W/O microemulsions have been calculated from the IR spectroscopy data, and the variations of the effective headgroup area of the AOT molecule have first been explained according to the populations of two kinds of conformations with different water content.