Abstract The Ilkovic constant was determined for zinc and zinc—amino acid complexes by using differential pulse polarography in 0.1 M sodium chloride—sodium diethylbarbiturate buffer at pH 7.4. A single well-behaved wave was obtained, except for cysteine and cystine for which useful calibration curves could not be obtained. The non-equilibrium dialysis of the amino acid complexes was studied in a hollow-fiber dialyzer. It is concluded that the dialysis step could be used in a method intended to measure available zinc but that the polarographic method needs further modification.