The kinetics and mechanism of electroreduction of tris-oxalate cobalt(III) complexes and oxalate cobalt(II) complexes at a dropping mercury electrode were studied with special attention to double layer effects for the electrode reaction of Co(III) to Co(II). Two irreversible waves corresponding to the electroreduction of Co(III) to Co(II) and Co(II) to Co(0) were observed. As a result of measurements in solutions containing exclusively Co(II) complexes, it was discovered that only aqua-complexes of Co2+ an involved in a slow electrochemical step, regardless of the oxalate concentration in the bulk solution. These complexes are formed by the fast splitting off of the coordinated oxalate ions from the originally predominant Co(II) oxalate complexes. The reversibility of the chemical reactions accompanying the electroreduction of these species also follows from the above measurements. This allows one to consider the aqua-complexes of Co(II) to be one of the products of the overall electrode reaction of the Co(III)/Co(II) couple. These generated particles, being positively charged, effectively participate in screening the electric field within the diffuse layer so that appropriate corrections for the double layer structure must be taken into consideration. The corresponding expressions for the above corrections are derived and their validity is shown by comparing theory and experiment.