Résumé : A method is presented to determine the Co-complexing capacity of seawater using cathodic stripping voltammetry (CSV) with adsorptive collection of Co (II) complexes with either nioxime or dimethylglyoxime (DMG). The method takes advantage of ligand competition for Co2+ exerted by the ligand added for CSV and natural complexing material in the seawater. Conditional stability constants for complexation of Co with nioxime and DMG in seawater of salinities between 1 and 35 were calibrated by ligand competition with EDTA. The 'detection window' of the method is determined by the Co-complexing ability of the added ligand, as expressed by the α-coefficient (=product of the conditional stability constant, KCoL, and the ligand concentration, CL) of the natural complexing ligand, L. With DMG as the added ligand, the detection window lies between 105.2 and 107.2 at a salinity of 35. The range of the detection window is shifted by varying the DMG concentration or by using nioxime for complexation of Co2+ DMG was used to determine the Co-complexing capacity in samples originating from the estuary of the Scheldt River and the Irish Sea. A variable fraction of 45-100% (average 73%) of dissolved Co was found to occur very strongly complexed. The ligand concentration was less than the Co concentration in most of the samples. Values for log KCoL were very high, and fell within in a narrow range of 15.6-17.5. The estuarine data suggest that the organically complexed Co behaves conservatively. © 1990.