The evolution of the impedance spectra of KC8 electrodes in 1 M LiClO4 + ethylene carbonate + dimethylcarbonate solution have been studied under open circuit voltage as a function of time. Additional studies have been performed before and after potassium deintercalation and in cycled electrodes. It has been found that KC8 slowly decomposes giving rise to a layer of insoluble KClO4 on the electrode surface that protects the compound from further decomposition. The interface may be simulated using two passivating layers in addition to the charge transfer and diffusion impedance. The same type of equivalent circuit, with the eventual inclusion of a third layer, may be used to simulate the spectra after K deintercalation, and during lithium intercalation.