The effect of Fe concentration on the Fe-induced self-discharge of electrochemical capacitor carbon electrodes in aqueous H2SO4 is presented. With an Fe-free system, the positive electrode self-discharges via an activation controlled self-discharge mechanism, while the negative electrode self-discharges with a diffusion control profile. This highlights that the self-discharge mechanism on each electrode of an electrochemical capacitor is likely different, and should be examined in a three-electrode (half cell) setup. It is shown that Fe concentrations up to 10−5 M can be tolerated with no enhancement of self-discharge on the positive electrode. Whereas the negative carbon electrode can withstand Fe concentrations of 10−3 M without self-discharge increase. Additionally, it is shown that the diffusion controlled Fe-induced self-discharge (at concentrations at and above 10−4 M on the positive electrode) occurs primarily on the external surface of the porous electrode, and the carbon surface inside the pores does not participate in self-discharge. This is used to explain why the Conway diffusion model for self-discharge, derived for semi-infinite diffusion to a planar electrode, can be used to fit the self-discharge process on porous electrodes.