Abstract Two methods that have now been used routinely for several years for on-line monitoring of cobalt in zinc plant electrolyte by adsorptive stripping voltammetry (ADSV) are described in which interference from the very high background zinc concentration is overcome. The first method combines ADSV of cobalt as its dimethylgloxime complex at a hanging mercury drop electrode with in situ matrix exchange. This method utilises a bottom-drain flow-through cell to overcome problems associated with the high density of zinc electrolyte and enables cobalt to be determined down to 9 μg 1 −1 in plant electrolyte solutions which contain 150 g 1 −1 zinc. The method is therefore suitable for low-purity electrolyte. However, in highly purified zinc electrolyte the matrix exchange technique lacks the required sensitivity for cobalt determination and a second technique known as catalytic differential pulse ADSV is employed which has a detection limit of 0.25 μg 1 −1 cobalt in zinc plant electrolyte. The catalytic technique again utilises a bottom-drain flow-cell, but involves formation of the α-benzil dioxime complex rather than that with dimethylglyoxime. Addition of nitrite enhances the cobalt signal and eliminates the need for matrix exchange. Both methods have been adapted foruse in on-line voltammetric analysers at Pasminco Metals-EZ. The condition used for efficient removal of cobalt during the purification process almost invariably result in removal of nickel to levels where possible interference from the presence of high nickel concentrations does not occur in the practical situation.