The redox behavior and color of cobalt hexacyanoferrate films depend on the nature of the counter-cations which are sorbed from the aqueous supporting electrolyte into the system during reduction. Whereas cobalt(II) hexacyanoferrate(II) is olive-brown in the presence of hydrated K+ or Cs+ ions, a green color is produced upon incorporation of larger cations (hydrated Na+ or Li+). The "normal" system (Co2FeII)-Fe-II(CN)(6) . nH(2)O, which is free of counter-cations, is deep green. These color changes are correlated with the thermochromic transformation of (K2CoFeII)-Fe-II(CN)(6) . nH(2)O from olive-brown to green upon heating above 61 degrees C. It seems that, at elevated temperatures, (K2CoFeII)-Fe-II(CN)(6) undergoes reorganization to the solid solution (Co2FeII)-Fe-II(CN)(6) . K4FeII(CN)(6) which contains the green "normal" phase. An analogous solid solution is presumably also formed upon exposure of (K2CoFeII)-Fe-II(CN)(6) film to electrolytes containing larger hydrated Li+ or Na+ cations. The voltammetric behavior in these electrolytes is consistent with the existence of such structures. The color of cobalt(II) hexacyanoferrate(II) is linked to the extent of aquation of the interstitial Co(II) ion, which is likely to be less hydrated in "green" systems (existing at higher temperatures or as "normal" phases free of structural hydrated counter-cations). In potassium electrolyte, cobalt(II) hexacyanoferrate(II) is electrochromic and becomes purple-brown upon oxidation.