Ultrafiltration (UF) of cobalt(II) and nickel(II) in the presence of a micelle-solubilized hydrophobic ligand has been investigated, using polyoxyethylene nonyl phenyl ether with 10 ethylene oxide units (PONPE10) and 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (EHPNA) as a nonionic surfactant and an extractant, respectively. In aqueous media, the metals are entrapped within the surfactant micelles containing the extractant, and are effectively rejected by UF membranes with their consequent concentration in the retentate solution. The selective rejection of Co(II) over Ni(II) can be attained; the separation is enhanced with increasing solution pH and the extractant concentration. On the other hand, it was found that sodium dodecyl sulfate (SDS) as a surfactant and di-2-ethylhexyl phosphoric acid (D2EHPA) as an extractant give no selectivity between these heavy metals in the same operation. The effects of the surfactant concentration, applied pressure, salt addition and the membrane geometry on the rejection were also examined in the EHPNA/PONPE10 system.