Abstract: The extraction of cobalt (Ⅱ) and magnesium with di (2-ethylhexyl) phosphoric acid (HDEHP) in kerosine from 0. 1Msodium chloride solution has been studied at 25±1℃. The experimental results indicate that the reaction of extraction of Co(Ⅱ) and Mg(Ⅱ) with HDEHPis a cat-ionic exchange process.The general equation for extraction equilibria can be represented as  The method for calculating the free concentration of the extractant in the organic phase has been discussed.