This work contains the results of research on the mechanism of the shuttle self-discharge determined by Fe2+ and Fe3+ ions in the electrolyte of a heterogeneous electrochemical supercapacitor (HES) of a PbO2 parallel to H2SO4 parallel to C system, as well as the results of the measurements of the effect of the shuttle self-discharge current on the energy, capacity, and operation parameters of HES capacitors. It is shown that the presence of the iron ions in the electrolyte of a HES capacitor brings about shuttle self-discharge, which continues until the voltage decreases to the value of 1.05 V. The shuttle mechanism is related to oxidation and reduction of Fe2+ and Fe3+ ions in the positive and negative electrodes, respectively. An analytical expression is obtained for the calculations of the shuttle self-discharge current determined by different types of impurity ions in the electrolyte of HES capacitors. Analysis is made with respect to the obtained experimental and theoretical data of the self-discharge current, energy, and capacity parameters, as well as their dependence on the concentration of iron ions in the electrolyte, the separator's thickness, and modes of the capacitors' charge. It is established that the processes of oxidation and reduction of iron ions take place in the near-surface layers of the capacitor electrodes. It is shown that the linear increase of the separator thickness does not bring about any linear decrease of the shuttle self-discharge current determined by Fe2+ and Fe3+ ions. (c) 2007 The Electrochemical Society.