The solvent extraction of 12-phosphotungstic acid, also known as 12-tungstophosphoric acidH3PW12O40, the so-called Keggin heteropolyacidby 0.73M (20%v/v) tri-n-butyl phosphate (TBP) in n-octane under conditions comparable to those used previously for the extraction of conventional inorganic mineral acids is described. A simplified phase diagram for the pentanary system comprised of H3PW12O40, HNO3, H2O, TBP, and n-octane reveals an extremely low initial concentration of H3PW12O40 (1.1mM) at the LOC (limiting organic concentration) condition, far lower than the most effective third-phase-forming inorganic acid, namely HClO4. The results from small-angle neutron scattering (SANS) indicate that the interparticle attraction energyU(r) calculated through application of the Baxter sticky sphere model to the SANS data at the LOC conditiondoes not approach the -2 kBT value associated with phase splitting in previous studies of TBP third-phase formation. The third-phase formation model based on attractive interactions between polar cores of reverse micelles, successfully developed for TBP and other extraction systems does not apply to the extraction of H3PW12O40. Rather, the separation of a third-phase from the TBP organic phase stems from the limited solubility of the heavy and highly polar H3PW12O40-TBP species in the alkane diluent.