The polarographic behaviour of cobalt(II) has been studied with respect to the composition of an aqueous solution of acetylacetone, using various polarographic techniques. The electrode process in pure perchlorate solution as well as in the presence of acetylacetone has been shown to be irreversible. The transfer coefficient, α=0.30, and the charge number of the reducible species, z =+2, are found to be valid for the reduction of cobalt(II) in pure perchlorate solution and in an aqueous solution of 10 −2 M acetylacetone at pH<4.5. In aqueous solutions of acetylaceton of pH<4.5, the transfer coefficient was: α=0.65 and the charge number z =+1; these values correspond to the reduction of the cobalt(II) monoacetylacetonato species. The double wave behaviour of cobalt(II) observed in the presence of acetylacetone at concentrations of the supporting electrolyte above 0.1 M is explained by the effect of electrostatic forces in the electrode double layer. In solutions where the ligand concentration is above 10 −4 M , the limiting current has a kinetic character caused by a slow dissociation of cobalt(II)diacetylacetonato to the monoacetylacetonato complex. The rate constants of the chemical reaction have been calculated: k d = ( 30.9 ± 0. I ) sec − 1 k r = ( 2.6 ± 0. I ) I O 5 mole − 1 l sec − 1