In the cobalt (Ⅱ)-dimethylglyoxime-NH_3-NH_4Cl (pH 9) system, me complex Co(Ⅱ)A_2exhibits a sensitive polarographic wave. The mechanism of this catalytic wave has been investigatedby linear potential sweep voltammetry, cyclic voltammerty and anodic stripping voltammetry. Theexperimental evidences showed that a zero-valence “active cobalt” or its complex formed during theirreversible reduction of Co(Ⅱ)A_2, which is adsorbed on the mercury electrode surface, and simul-taneously DMG is catalytically reduced by this “active cobalt”. The mechanism of this system withthe conflicting explanations of a catalytic hydrogen wave or only adsorptive complex wave is dis-cussed. In the cobalt (Ⅱ) -dimethylglyoxime-NH_3-NH_4Cl (pH 9) system, me complex Co (Ⅱ) A_2exhibits a sensitive polarographic wave. The mechanism of this catalytic wave has been investigated by linear potential sweep voltammetry, cyclic voltammerty and anodic stripping voltammetry . The experimental evidences showed that a zero-valence “active cobalt” or its complex formed during their reversible reduction of Co (II) A_2, which is adsorbed on the mercury electrode surface, and simul- taneously DMG is catalytically reduced by this “ active cobalt ”. The mechanism of this system with the conflicting explanations of a catalytic hydrogen wave or only adsorptive complex wave is dis-cussed.