Precise determinations of dielectric constants were carried out for Aerosol OT reversed micelles in cyclohexane at a frequency of 1 MHz and at a temperature of 298.15 K for molalities of AOT from 0 to 0.04 mol kg(-1). The ratio of the amount of water, r, to that of AOT was varied from 0 to 25 and the apparent dipole moments for the mixed solute (AOT + r . H2O) were calculated. Discontinuous change in the limiting dipole moment with increasing r was found and was related to the characteristic of water to bind to the head group of AOT. The process of forming W/O microemulsion and the molecular interactions in the micellar core are discussed in terms of the hydration of AOT.