Isomers of [Ni(CRH)](2+) have been isolated and studied by d.c. and a.c. voltammetry on mercury electrodes in aqueous solution. The configurational isomers of [Ni(CRH)](2+) were found to differ in their stability in the solution and the isomerization process was monitored voltammetrically. The adsorptivity of the isomer A on mercury electrodes was found to be stronger than that of the isomer B, while no differences were seen for the adsorbed Ni(I) complexes. The solution resident [Ni(CRH)](+) complexes are more labile than the corresponding Ni(II) compounds, hence upon reduction the isomer B undergoes fast transformation into the more stable isomer A. The electroreduction of [Ni(CRH)](2+) in the presence of CO and CO2 is described and the influence of the isomeric form used in the processes is discussed.