The extraction of cobalt ions from dilute aqueous solutions was investigated by ultrafiltration with the help of poly(acrylic acid) sodium salt (PAASS). Polysulfone and polyethersulfone hollow fiber ultrafiltration membranes were employed in this process. The kinetics of complexation reaction was studied for PAASS with Co. Results showed that, under a large excess of PAASS, it takes 65, 55 and 40 min at pH 5, 6 and 7, respectively, to get the equilibrium of complexation. The reaction kinetics can be described by a pseudo-first-order equation. Then, the effects of various parameters on the extraction of Co were examined in detail. Results indicated that loading ratio, pH value and low-molecular competitive complexing agent affect significantly cobalt rejection coefficient . Furthermore, a concentration experiment was carried out at pH 7. With increasing volume concentration factor, membrane flux declines slowly, and value is always about 1. The concentrated retentate was used further for a decomplexation experiment. The decomplexation ratio of cobalt-PAASS complex reaches as high as 90.1%. After the decomplexation step, a diafiltration experiment was performed at pH 2.5. Cobalt ions can be extracted satisfactorily from the retentate, and a purified PAASS is obtained.