Polymer enhanced ultrafiltration (PEUF) technique economically enables separation and/or retention of various heavy metal ions from aqueous solutions for environmental and engineering problems. Polyethylenimine as the water-soluble complexation agent utilized to interact with heavy metal ions of Cd and Ni as cations in ultrafiltration experiments were modeled with protonation data to understand the mechanism. Test runs showed that an enrichment coefficient (the ratio of Cd over Ni in permeate) up to 10 was obtained at pH 4.2 in the temperature range of 20-22^oC, whereas full retention of both cations is possible above pH 5.0 and those results are highly compatible with the model developed.