The retention behavior of copper(II)-ethylenediaminetetraacetic acid (EDTA) chelates in dilute aqueous solutions ([Cu2+] = 0.5-4 mM) by ultrafiltration was studied with the help of a basic polyelectrolyte, polyethylenimine (PEI). The regenerated cellulose Amicon YM10 and YM30 were used as ultrafilters. Experiments were performed in a stirred batch cell as a function of aqueous pH (2-10), the concentration ratios of EDTA to copper ions (0.5-4) and of PEI to copper ions (2.5-20), and the applied pressure (0.68-3.40 atm). The best conditions for rejection coefficient greater than 97% equivalent to 7 ppm copper chelates were obtained. The effect of inorganic salts such as NaCl, CaCl2, and Na2SO4 on the retention was also examined. In addition, the membrane fouling during ultrafiltration was analyzed by modified fouling index. The regeneration method of PEI from the retentate is finally discussed.