In this study a polymer inclusion membrane (PIM) for the extraction and separation of uranium (UO(2)(2+)) from acidic sulfate solutions is detailed. Poly(vinyl chloride) (PVC) based PIMs containing the following commercial extractants were screened for their ability to extract uranium: Alamine 336, Cyanex 272, Aliquat 336, di-(2-ethylhexyl) phosphoric acid (D2EHPA), and tri-n-butyl phosphate (TBP). The membrane containing 40% (m/m) D2EHPA is shown to be the most effective in the quantitative extraction and back extraction of uranium using sulfuric acid solutions. Facilitated transport of uranium across the membrane against its concentration gradient is demonstrated with uranium fluxes as high as 4.85 x 10(-7) mol m(-2) s(-1). A new method for determining the stoichiometry of extracted complexes in both membrane and solvent extraction systems has revealed the composition of the UO(2)(2+)-D2EHPA complex to be UO(2)center dot A(2)center dot(HA)(2) where HA and A(-) are D2EHPA and its conjugated base, respectively. (C) 2010 Elsevier B.V. All rights reserved.