The state of aggregation of metal salts, M(n+) (DEHP(-))(n), of the organophosphorous acid extractant bis(2-ethylhexyl) phosphoric acid (HDEHP) has been examined by small-angle neutron scattering (SANS) and viscosity measurements in cyclohexane. A range of di- and trivalent counterions (M(n+)) were studied as a function of concentration and water content, where M(n+) = Ca2+, Co2+, Ni2+, Cu2+ Mn2+, Al3+, and Cr3+. The extent of water uptake by di- and trivalent salts as defined by omega = [H2O]/[M(n+) (DEHP(-))(n)] was found to be low such that only hydrated reversed micelles were formed. The results support formation of rod-shaped reversed micelles by all divalent metal ion salts with the SANS I(Q) well-represented by a rod form-factor with radii in the range 7.5-9 Angstrom. The length of the rod-shaped reversed micelles formed was strongly dependent upon the nature of the counterion with fitted values between similar to 80 Angstrom (M(n+) = Ni2+) and similar to 300 Angstrom (Mn2+). Although the shape and size of reversed micelles were found to be independent of surfactant concentration in the range c = 0.04-0.08 g cm(-3), Cu(DEHP)(2) was found to undergo aggregation resembling critical micelle concentration like behavior into rod-shaped micelles at c similar to 0.02 g cm(-3). With the exception of Ni and Cu the state of aggregation of all salts examined was unaffected by addition of water. In contrast, the trivalent-metal-ion salt, Al(DEHP)(3) is present in solution either as monomer or as small spherical aggregates of low aggregation number.