This paper presents voltammetric studies on the electroreduction of 4-amino-3, 5, 6-trichloropicolinic acid (picloram) on mercury electrodes. At concentrations higher than 2x10(-4) M, two prepeak systems were found in the potential ranges -0.6 to -0.8 V (system I, close to the main reductions peak) and 0 to -0.4 V (system II). Under these conditions, these prepeak systems can be attributed to the weak adsorption of reactant and the strong adsorption of the product at the electrode surface. Above pH 3, prepeak systems I and II were not found in the voltammograms, and that could be related to the acid-basic equilibrium corresponding to the disssociation of the carboxylic group. The values of the adsorption half peak width, the separation between the adsorption prepeak and the diffusion peak, the ratio between the intensities of these peaks and the experiments performed in aprotic media, showed that the species responsible for the appearance of the peak system II must be the uncharged form of picloram, whereas that responsible for the appearance of system I must be the picloram protonated at the pyridinic nitrogen.