We investigated how the increase of NaOH concentration in the water pool affects the phase behavior and size of Na-AOT-based W/O microemulsions. A mixture of NaOH aqueous solution/Na-AOT/isooctane was emulsified by changing the NaOH concentration C-NaOH at a water content w(0) = [water]/[AOT(-)] = 10. FT-IR measurements demonstrated that the amount of water in the upper phase of the emulsified mixtures decreases with increasing C-NaOH to form a lower phase, whereas the amount of AOT- in the upper phase remains nearly constant. Substituting NaCl for NaOH resulted in the corresponding phase behavior, indicating that the phase separation is due to the increase of not pH but Na+ concentration. Possible mechanism for the phase separation is discussed by focusing on screening effect of Na+ counterions on electrostatic repulsions between AOT(-) polar headgroups. The apparent water content w(0)', estimated using the amount of water and AOT- in the upper phase, decreases with increasing C-NaOH. DLS measurement showed that the upper phase contains single-nanometer W/O microemulsions and the size decreases with increasing C-NaOH with progress of the phase separation. Measurements of FT-IR and DLS were also carried out for W/O microemulsions prepared with pure water by changing the water content w(0). Wavenumber positions of hydroxyl and sulfonate groups were determined by FT-IR measurements. Interestingly, the dependence of wavenumber positions and size on w(0)' agrees with those on w(0), which indicates that variation of W/O microemulsion's property with progress of the phase separation corresponds to the variation with decreasing the water content to prepare W/O microemulsions with pure water. (C) 2009 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.