The electrochemical redox behavior of thin nickel-iron cyanide (NIC) film electrodes was investigated. Thin films of NIC prepared electrochemically on ITO glass were polarized anodically and cathodically in aqueous solutions of NaCl and KCl, and their absorption spectra were measured in the visible and near-IR regions. Using a semi-infinite model, apparent diffusion coefficients D-APP of the alkali ions were measured by the potential step method. The NIC-modified electrode showed reversible cyclic voltammograms without early cycling. The absorption spectrum of the NIC film in the oxidized state was almost the same as that in the reduced state, i.e. the film was almost transparent in both states. When the film thickness was around 10(-8) m, D-APP in the NIC film was on the order of 10(-14) m(2) s(-1). The anodic and cathodic diffusion behavior of sodium and potassium ions was estimated and is discussed from the viewpoint of morphology of the film and hydration of alkaline ions.