We have discovered a novel procedure that yields extremely stable ruthenium-modified indium-hexacyanoferrate(II/III) film electrodes. The process includes electrochemical deposition of the indium(III)-hexacyanoferrate (InHCF) film and its subsequent voltammetric conditioning in 1 mM RuCl3. The resulting inorganic film was characterized by electrochemical, piezoelectric microgravimetric, and surface spectroscopic techniques. Frequency changes due to mass increase of the indium-hexacyanoferrate film onto the platinum-crystal working electrode were correlated with the incorporation of ruthenium, which imparts an extensive cross-linking between hydrate InHCF particles through dinuclear [Fe, Ru] oxo (-O-) and cyanide (-CN-) bridges. InHCF films prepared by this procedure and analyzed by XPS support the conclusions drawn from EQCM results. Upon modification, the inorganic film exhibits remarkable electrochemical stability, there being no observed decrease in peak current signal after at least 8600 cycles in the potential window from 0.0 to +0.9 V vs Ag\AgCl. Evidence is also provided for the incorporation of K+ along with one water molecule during the reduction process. The cation selectivity of the ruthenium-modified InHCF film is K+ > Na+ > Li+, Csf >> NH4+, Rb+, and the fact that the original voltammetric profile was restored when the Ru-modified InHCF film electrode was cycled in K+ further confirms its greater selectivity.