Both ionic liquids and water are typical green solvents. In this work, the phase behavior of the ternary system consisting of ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF(6)), TX-100, and water was determined at 25.0 degrees C. The water-in-bmimPF(6), bicontinuous, and bmimPF(6)-in-water microregions of the microemulsions were identified by cyclic voltammetry method using potassium ferrocyanide K4Fe(CN)(6) as the electroactive probe. Dynamic light scattering (DLS) and the UV-vis method were used to characterize the microemulsions. It was demonstrated that the hydrodynamic diameter (D-h) of the bmimPF(6)-in-water microemulsions is nearly independent of the water content but increases with increasing bmimPF(6) content due to the swelling of the micelles by the ionic liquid. The UV-vis further confirmed the existence of water domains in the water-in-bmimPF(6) microemulsions; and the salt potassium ferricyanide K3Fe(CN)(6) could be dissolved in the water domains.