An analytical procedure for chemically measuring the complexing capacity (ability of a water to complex metal ions) of natural water is developed and tested. The procedure can be used in evaluating the role of natural complexing agents in the transport, toxicity, and availability of trace metal ions in natural waters. Three methods were evaluated, two based on micro scale complexometric titration using anodic stripping voltammetry, and a third, the recommended procedure, involving chemical conversion of the ligands (substances capable of forming metal complexes with metal ions) in the sample to the corresponding cobalt (III) complex. Differential pulse polarography was used to monitor the decreases in cobalt (III) concentration after production of the cobalt (III) complexes and hence the complexing capacity of the sample. This procedure has a detection limit of 0.6 micromoles of ligand per liter.