Sodium extraction and water uptake from a sodium chloride aqueous solution by a di (2-ethylhexyl) phosphoric acid (D 2 EHPA)/n-heptane system was experimentally studied. The behavior of water uptake was classified into three regions based on the pH dependence of the water content, W-0. In the region of pH 1-4 (Region 1), the molar ratio of sodium in the organic phase to total D 2 EHPA, E-Na, increased when pH was higher. However, W-0 was low and did not depend on pH. In the pH region over 6 (Region 3), W-0 was high and showed no pH dependence. D 2 EHPA existed as sodium di(2-ethylhexyl) phosphate (SD 2 EHP) and formed reversed micelles in this region. In the region of pH 4-6, both E-Na and W-0 increased with the rise in pH. This region is thought to be a transition state from D 2 EHPA-Na complex to the formation of the reversed micelles. The behavior of the water uptake into the organic phase at the Region 1 and 2 was analyzed by the extraction equilibrium theory. It was found that the pH dependence of the water content under the low loading ratio could be explained using the model that water molecules coordinated into D 2 EHPA-Na complexes.