The multicomponent system consisting Of poly(sodium 4-styrenesulfonate) (PSS), iminodiacetic acid (IDAA), and Cu(NO3)(2) in water at PH 4 was studied by means of ultrafiltration at low and relatively high ionic strengths. Under the, experimental conditions, IDAA is negatively charged, and is slightly retained in ultrafiltration experiments in the presence of the water-soluble polyelectrolyte PSS. The positively charged copper ions are strongly retained by PSS. Due to the capability of IDAA to form stable complexes with Cu2+ ions, the resulting blue 1:1 complex between IDAA and Cu2+ is not retained by PSS, but is eluted from the ultrafiltration cell at PH 4.