During electroreduction of dicyanoargentate two limiting currents were observed in the cathodic voltammogram. Three mechanisms were identified for the first limiting current : (i) slow homogeneous chemical reaction; (ii) slow heterogeneous chemical reaction; (iii) partial passivation of the electrode surface. The features of voltammograms obtained in solutions of varied viscosity and the effect of the strongly absorbing species on these features made it possible to eliminate the first mechanism. The evidence for the third mechanism comes from electrochemical impedance data : a linear relationship between the real component of this impedance and the frequency to the power -0.5 was observed, whereas in the case of a heterogeneous reaction the frequency should be raised to the power -2. A rough evaluation of the rate of surface passivation was performed from chronoamperometric measurements and from features of the change of the real component of impedance with time.