A Fourier transform infrared study of the states of water included in reverse micelles made from Aerosol-OT (AOT) in isooctane is carried out for the whole sequence of alkali metal counterions as a function of the water content. The decomposition of the aqueous pool of these micelles into “bound” and “free” water is assumed. Two independent methods are applied for the purpose of interpreting the shape and the intensity of the water OH stretching vibration band. The modelization of the OH stretching band by three Gaussian components is retained as the most efficient decomposition and as an adequate smoothing method. To determine the number of water molecules bound per AOT polar head, three water adsorption models are tested for their ability to fit of the experimental data, to reproduce the effect of the AOT concentration, and to indicate the influence the counterion forming the ion pair with AOT. A Langmuir adsorption isotherm model satisfies best these criteria. The results suggest that the hydration process concerns exclusively the AOT molecule without any marked interference depending on the counterion. The maximum hydration number of AOT determined by this treatment is between 2 and 3. The estimated standard free energy of the AOT polar head hydration is about 2.5 kJ/mol.