The ammonium salt of the metal ion extractant bis(ethylhexyl) hydrogen phosphate (HDEHP) has been found to stabilize water-in-oil (w/o) microemulsions over a wide range of omega (=[H2O]/[NH(4)DEHP]) and temperature. Such extensive solubilization has not been observed for Gp I, II, or III metal salts of HDEHP. In the absence of added water, SANS measurements show the formation of rod-shaped reversed micelles which progressively decrease in length as waters added in the range omega < 5. At constant omega the rod length increases with surfactant concentration. For omega > 5 spherical droplet w/o microemulsions are formed with the droplet size increasing in proportion to omega. Analysis of SANS data gives the head group area of NH4-DEHP in the surfactant layer of larger spherical microemulsion droplets as 63 +/- 3 A(2).