Carbonic anhydrase (CA) is inactive unless associated with zinc, with possible substitution by cobalt. In this work, the complexation of zinc by CA was determined in sea-water using cathodic stripping voltammetry (CSV) with ligand competition. The zinc was found to be released from the CA over a period of 3 h when equilibrated with a competing complexing ligand and the complex was re-formed with the CA when zinc was added. A value of 8.90 ± 0.27 was found for logKAZnCA where KAZnCA is the conditional stability constant for the complex of Zn2+ with CA in pH 8 sea-water. A value for the molecular weight of CA was calculated from its equivalent ligand concentration (in nM) obtained by titrations with zinc at various CA concentrations (1–4 mg l21). The value found (34740 g mol21) for the molecular weight is consistent with values found previously by other methods (29000–31000 g mol21) confirming that the stoichiometry of the complex between zinc and CA is 1+1. This work confirms that the zinc–CA complex is reversible and that the interaction between zinc and CA can be determined using CSV with ligand competition.