The anodic depolarization wave of Hg +2 by complexone (ethylenediaminotetraacetic acid) has been found to be proportional to the concentration of complexone (10 -4 to 10 -3 mole/liter, pH =6.4 in buffered medium). Titrationc of complexone by Mn +2, Co +2, Ni +2, Cd +2, Mg +2, Zn +2 gave, at the same pH of 6.4, normal amperometric curves. On the other hand, alkaline-earth cations, the complexes of which are less stable than the aforementioned ones, gave abnormal amperometric curves in that the height of the wave observed during titration is much larger than the one corresponding to the concentration of the complexone that is not involved in the formation of the complex. These results can nevertheless be interpreted in a satisfactory way by assuming that the measured current is determined on the one hand by diffusion and on the other hand by the rate of dissociation of the complexes in the vicinity of the electrode. The rate constant is greater the less stable the complex.