In the experiment, we prepared metal electrodes with sub-micron to nanometer for the scanning electrochemical microscopy to study the kinetics of Ru(NH 3) 6 3+/2+ redox system. The rates of reduction of Ru(NH 3) 6 (NO 3) 3 were found essentially independent of the nature of the electrode material, but mildly dependent upon chloride ion concentration. We observed that the 'outer-sphere' reactant of Ru(NH 3) 6 (NO 3) 3 reduced at relatively slower rates in low-chloride solutions. This phenomenon appears to be compatible with the expected variations in a ligand-bridged mechanism in which the electrostatic repulsions among the specifically adsorbed chloride anions that dominate would have an adverse influence on the adsorption of inner-sphere cationic reactants and thus cause a decrease in the reaction rate. However, as 0.1 M supporting electrolyte KCl was added into the target solution, a significant increase in the reaction rate was measured as a result of the ionic double-layer effect. Cationic complexes will experience the same electrostatic attraction from adsorbed chloride anions as will outer-sphere reactants. It is suggested that the reaction pathway of Ru(NH 3) 6 3+ reduction can change from the anion-bridged inner-sphere to the outer-sphere with increasing chloride ion concentration up to 0.1 M. [ABSTRACT FROM AUTHOR]