The high electrocatalytic activity of the Ni(cyclam)2+ complex for the reduction of CO2 at mercury electrodes originates in the strongly adsorbed Ni(cyclam)+ complex produced by reductive adsorption. The catalytic activity is severely diminished in the presence of CO at unstirred mercury electrodes. As CO is the primary product of the reduction of CO2, this behavior limits the long-term effectiveness of the catalyst. The origin of the decrease in activity is proposed to be an insoluble complex of Ni(0), Ni(cyclam)CO, which is formed during the reduction of CO2. A mechanistic scheme which accounts for the electrochemical responses obtained at mercury electrodes in solutions containing Ni(cyclam)2+ and CO2 is presented. It shares some features with previously proposed schemes but also contains some new interpretations and proposals.