The ultrafiltration technique evaluates the interactions of water-soluble polymers with metal ions. Aqueous solutions containing poly(sodium 4-styrenesulfonate) (PSS), Cu(NO3)(2), NaNO3, and iminodiacetic acid (IDAA) are examined by this technique. Cu2+ undergoes complex formation with IDAA and intreracts electrostatically with PSS. On the other hand, Na+ ions are in competition with Cu2+ for the electrostatic binding to PSS. The solutions are ultrafiltered keeping the ionic strength constant, so their compositions are allowed to change continuously. The concentration of Cu2+ bound to the polymer showed an exponential decay during filtration. The concentration of Cu2+ bound to the polymer before ultrafiltration is calculated by extrapolation. The concentration of the different species in solution is proposed as a function of the filtration factor.