The complex cobalt hexacyanoferrate was immobilized onto a silica gel surface, chemically modified with tin(IV) oxide. The formal potential of the attached electroactive species was dependent on the supporting electrolyte used : NaNO3 = 0.39 V, KNO3 = 0.50 V and NH4NO3 = 0.53 V. A Nernstian behavior was observed for KNO3 and NH4NO3 and a quasi-Nernstian one for NaNO3 when they were used as the supporting electrolyte. The LiNO3 did not generate a reversible current peak since the hydrated ionic radius is larger than the zeolite-type cavity of the surface mixed valence complex.