The removal of divalent copper and zinc ions from an aqueous sulfate solution with polyacrylic acid (PAA) was examined by a batch complexation-ultrafiltration process. Factors affecting the rejection and permeate flux such as the membrane pore size and the applied pressure were investigated. In addition, on the basis of the known chemical equilibrium constants obtained from a mono-ionic system, a rejection mechanism taking into account the formation of soluble metal hydroxy complexes was proposed to predict the rejection coefficient of metal ions and the separation factor in a bi-ionic solution.