The solution structures in the-three-component system sodium bis(2-ethylhexyl) phosphate (NaDEHP)/ n-heptane/water have been investigated by dynamic and static light-scattering, electrical conductivity, and P-31-NMR spectroscopic and viscometric measurements. A transition in the physicochemical properties was observed over a rather narrow range of W-o (molar ratio of water to NaDEHP) values. Rodlike reversed micelles and swollen rodlike reversed micelles form when W-o < 4. Beyond this W-o range, phase separation occurs at low NaDEHP concentrations, and homogeneous one-phase microemulsions form at high NaDEHP concentrations. The transition for high NaDEHP concentrations has the following characteristics : the NaDEHP/n-heptane solutions are essentially nonconducting and highly conducting below and above W-o approximate to 4-7, respectively, and the dependence of viscosity on water content changes from positive to negative at W-o approximate to 4. Furthermore, the line width of the P-31-NMR absorption spectra is initially very broad, and it decreases sharply with increasing water content and appears to attain a constant low value at W-o > 7. It is suggested that the n-heptane-continuous solution of water-swollen reversed micelles transforms to a bicontinuous microemulsion when W-o increases beyond W-o approximate to 4. The bicontinuous microemulsion region ranges between two local viscosity maxima at W-o approximate to 4 and W-o approximate to 100, and an oil-in-water (O/W) microemulsion exists when W-o > 100. Local dynamic domain structures consistent with the bicontinuous microemulsion behaviors are proposed and discussed.