The electrochemical oxidation of cobalt(II) (Co(II)) on Ag, a partial reaction of the electroless silver plating process, was studied in ammonia, ammonia + ethylenediamine (En) and En solutions (pH 9-12) by voltammetry using a rotating disc electrode and electrochemical quartz crystal microgravimetry (EQCM). In NH3 + (NH4)(2)SO4 solutions Co(II) (0.01 mol l(-1)) oxidation takes place simultaneously with Ag dissolution. The Co(II) oxidation on a non-dissolving Ag electrode is observed only at higher concentration (0.1 mol 1(-1)) at pH 10-11. The addition of En does not accelerate Ag dissolution but enhances Co(II) oxidation and shifts its region to more negative potential values by ca. 0.3 V. The En effect is explained by the formation of more easily oxidized and more active Co(II) complexes with En. The Co(II) oxidation current was found to be proportional to CoEn(m)(2+) (m = 2, 3) concentration and to depend on solution pH. The Co(II) oxidation rate in the presence of En is higher compared to that in ammonia solutions by a factor of 10-40.