Electrodes modified with cobalt(II) hexacyanoferrate films were synthesized in solutions of various sodium ion concentrations by consecutive cyclic voltammetry. In a 0.1 M NaNO3 electrolyte solution, the voltammetric response was characterized by a single redox couple, while in a 1.0 M NaNO3 electrolyte solution two distinct redox couples appeared. Film characteristics dependent on the hexacyanoferrate : cobalt(II) ratio were also investigated. The electrochemical and spectroscopic properties of these films are presented here. The electrocatalytic oxidation of NH3OH+, NO2- S2O32- and N2H4 at electrodes modified with cobalt(II) hexacyanoferrate films in 0.1 M and 1.0 M NaNO3 electrolytes are reported and compared with behaviours observed at electrodes modified with indium(III) hexacyanoferrate and iron(III) hexacyanoferrate films.