Phase equilibria and nonequilibrium structures in the sodium di-2-ethylhexyl phosphate (NaSDEHP)–decane–water system were studied. It was found that, at a certain component ratio, a microemulsion and a liquid crystal phase are present in the system. The phase diagram was plotted, describing two- and three-phase equilibria: NaSDEHP and water solution in decane–microemulsion, microemulsion–liquid crystals, NaSDEHP and water solution in decane–microemulsion–liquid crystals, and NaSDEHP and water solution in decane–liquid crystals. The viscosity of NaSDEHP microemulsion was measured. It was shown that the viscosity increases significantly with an increase in the ratio of sodium hydroxide and di-2-ethyhexyl phosphoric acid molar concentrations from 0.6 to 1.0. Formation of the third liquid (microemulsion) phase in the vicinity of the oil–water interface was found upon the transfer of NaSDEHP from one phase to the other.