Mechanism of electrode poisoning in the reduction of CO2 catalyzed by [Ni(CRH)](2+) and [Ni(cyclam)](2+) is studied by cyclic and stripping de voltammetry. In both cases the product of CO2 reduction is CO, which undergoes further reaction with the Ni(I) form of the catalyst. Comparison of the process in solutions saturated with CO2 and CO proved that the decay of catalytic efficiency was due to the formation of Ni(0) carbonyl compound blocking the electrode surface. Stripping voltammetry in diluted solutions of catalyst allowed to neglect bulk reactions of catalyst and revealed the transformation of adsorbed Ni(I) complex catalyst into the catalytically inactive Ni(0) carbonyl deposit. The latter is oxidized yielding a well developed anodic stripping peak. This signal was used to quantify the extent of harmful deposit formation and was shown to be distinctly larger in case of the [Ni(CRH)](2+) catalyst.