Electrochromic displays made from thin films of iron hexacyanoferrate (Prussian blue) deposited on tin oxide/glass substrates by electroreduction from solution are studied by x‐ray photoelectron spectroscopy (XPS). It was determined by XPS that the concentration of potassium ions in the film increased as the number of electrochromic switches the film had undergone increased. This was accompanied by a decrease in the electrochromic performance of the film. The incorporation of potassium ions into the Prussian blue lattice causes a structural reorganization of the lattice with the expulsion of ferric ions from the lattice. The expulsion of ferric ions results in a degradation of the film’s electrochromic properties. This agrees qualitatively with data collected by insitu optical spectroscopy, XPS, atomic absorption spectrometry, optical microscopy, and x‐ray diffraction. The rate of degradation can be reduced substantially by inclusion of an Fe(II)/Fe(III) containing solution in switching electrolyte. The role of ionic transport in the degradation mechanism is discussed.

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