Based on our previous work, performance of separation of Cd2+ by the CoAlPO4-5/polysulfone membrane was not as good as that of the other metal ions. This could be significantly improved by the addition of other metal ions into the Cd2+ solution. The metal ions added were those with larger hydrated radius than that of Cd2+. When Mg2+ and Ca2+ were added, the rejection rate of Cd2+ could be increased from about 50% ([Cd2+] = 50 ppm) to nearly 100% and the rate would increase with concentration of Mg2- and Ca2+. However, when the added ions were Fe2+ and Mn2+, although the performance of Cd2+ removal was still high, the rejection rate would inversely decrease with increasing concentration of Fe2+ and Mn2+. These phenomena were mainly attributable to the effect of hydrated ion and the effect of lowered pH value induced by the hydrolysis of the solution. As to the addition of Al3+, although the rejection rate would decrease with increasing Al3+ concentration up to 100 ppm, an abrupt increase was observed at 150 ppm, which was thought to be attributable to the concentration polarization effect caused by the formation of the high valence [Al-n(OH)(m)]((3(supi]n-[supi]m)+) complex.