The electrochemical behaviour and electrocatalytic activity of a ruthenium-modified cobalt-hexacyanoferrate (CoHCF) thin film electrode, grown electrochemically on a glassy carbon substrate, is described. The modification, accomplished by potential cycling a freshly prepared CoHCF film electrode in acidic 50 mM NaCl solutions (pH 2-3 with HCl) containing ruthenium(III)chloride. represents a very effective contribution to the maintenance of electrode stability. Indeed, whereas a continuous decay over a few hundred redox scans at the parent CoHCF was observed, the resulting Ru-modified CoHCF film electrode exhibits greatly improved stability and enhanced electrocatalytic activity. Fifteen thousand redox scans at 100 mV/s caused the peak current to decrease by only about 10% by virtue of the strong stability of the Ru-modified CoHCF film electrode. Such an excellent stability is likely imparted by the formation of an extended network of dinuclear [Fe, Ru] ore-bridged, and has been interpreted by the insertion of Ru-O- moieties within the hydrated microparticles of CoHCF. Some examples are also presented which illustrate the excellent electrocatalytic capability in acidic solutions (pH 3) of the ruthenium-modified film towards the oxidation of hydrazine, thiosulfate, and p-chlorophenol. The catalytic oxidation currents observed are proportional to the concentration of hydrazine up to about 5 mM with a sensitivity of 2.7 mu A mM(-1) mm(-2) and correlation coefficient of 0.999. (C) 1999 Elsevier Science S.A. All rights reserved.