It has been reported for a number of systems, such as Ni(OH)(2) or certain electronic conducting polymer films, that the charge stored in the electrode evaluated by ac impedance is significantly lower than that determined by cyclic voltammetry or galvanostatic discharge. The charges exchanged under low and large amplitude perturbations were investigated using a staircase potential program. The results obtained with a thin-layer alpha-Ni(OH)(2) electrode were compared with the responses computed from a simple redox model of a two-dimensional interface. The calculated data for the profile of the voltammetric peaks, their dependence on the sweep rate and the transient responses near the peak potential are in reasonable agreement with experiment. It is concluded that the discrepancy observed originates from the non-linearity of the electrode system induced by a sharp transition between the insulating and the conducting states.