The dialysis of nickel (II) issued from diluted aqueous solutions to hexane solutions of di 2-ethylhexyl phosphoric acid (D2EHPA) through a polysulfone ultrafiltration membrane has been investigated. When the pH of the feed solution is lower than 5, the rate of nickel transport is limited by the kinetic of the complexation reaction. At higher pH values, the rate of nickel transport can be limited either by the mass transfer of nickel in the aqueous phase or by that of the extracting agent, or the formed complex in the membrane. The mass transfer coefficients were determined.