The structure of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles and water-in-oil microemulsions in isooctane and cyclohexane has been investigated using vibrational spectroscopy. The formation of reverse micelles resulted in an increase in the interaction between the Na+ counterion and the -SO3- group, compared with the solid state. Hydration of reverse micelles in cyclohexane to form water-in-oil (w/o) microemulsions led to a shift in the wavenumber of the symmetric sulfonate stretching mode from 1064 to 1050 cm(-1) in the Raman spectrum and from 1051 to 1045 cm(-1) in the infrared spectrum. The difference between the wavenumber of the band maxima in the infrared and Raman spectra, which decreased from 13 to 5 cm(-1) after hydration, is attributed to correlation coupling. The splitting of the antisymmetric sulfonate stretching mode in the infrared spectrum, which produces two strong bands at ca. 1212 and 1248 cm(-1) in the reverse micelles, is also reduced upon hydration. These results are interpreted in terms of the increase in the separation between the Na+ counterion and the -SO3- group and an associated increase in the separation between the -SO3- groups of adjacent surfactant molecules at the micellar interface. The greatest changes in the interaction between the sulfonate moiety and the sodium counterion occurred below w(0) = [H2O]/[AOT] = 3, with only small band shifts in the spectra observed beyond w(0) = 6. No shifts were evident beyond ca. w(0) = 15. These results are consistent with a strong hydrogen-bonding interaction involving three H2O molecules located between the oxygen atoms of the -SO3- group and a Na+ cation, and the requirement of at least six H2O molecules per AOT molecule to completely solvate the head-group. In contrast, hydration produced only minor changes in the relative infrared intensities of carbonyl stretching modes at ca. 1735 and 1723 cm(-1). These observations reflect changes in the relative populations of different conformations about the acyl C-C bond and an increase in the degree of rotation, which accompanies the increase in the average area occupied by the head-group upon hydration.