Persistent questions regarding the electrochromic ferric-ferrocyanide chromophore, Prussian blue (PB), are addressed and definitively resolved by detailed chemical and energy-dispersive X-ray analyses and spectroscopy: (i) What is the nature of the initially deposited film? (ii) Restructuring of freshly deposited PB that occurs in the first voltammetric cycle in KCl(aq): why is only one-third of the maximum possible K+ taken up? (iii) Is the presence of KCl in the electrodeposition solution beneficial or destructive? and (iv) What underlies the appreciable spectroscopic shifts of the charge-transfer peak at similar to700 nm, that depend on which alkali-metal ion M+ is incorporated in PB? The KCl-cycled PB after electrodeposition is shown to be K0.33+Fe0.332+Fe3+[Fe(CN)(6)(4-)] in a mechanism explaining the limited (0.33) uptake of K+. Similar compositions are found with other M+ except Cs+, which interacts exceptionally. (C) 2003 The Electrochemical Society.