Electrodes used for capacitive deionization (CDI) are prone to self-discharge reactions, in which their electrochemical potentials can drift over time under open-circuit conditions. These self-discharge reactions inhibit energy recovery during the cell discharge step when operating CDI cells, which increases the amount of energy required to deionize water. Self-discharge reactions for porous carbon electrodes are well-documented in the literature, where they have been attributed primarily to parasitic faradaic reactions and charge redistribution reactions in pore networks. Little work, however, has examined how and why self-discharge reactions occur for electrodes that intercalate cations into their structures. Here, we report that nickel hexacyanoferrate, a Prussian blue analog, undergoes significant self-discharge under open-circuit conditions in neutral saline solutions, which adversely impacts its use in deionization cells. We examined how the charging history, charging process, and electrolyte composition influenced self-discharge. Preliminary evidence indicates that self-discharge is largely a consequence of charge redistribution within the crystal lattice. Our findings provide insights into the feasibility of using Prussian blue analogs as electrode materials for deionization as well as caution in how salt absorption capacities of electrode materials are quantified.