Nanostructured molecular film containing the (mu-hydroxo)bis(mu-carboxylato) diruthenium(III) units, [Ru-2(III)(mu-OH)(mu-CH3COO)(2)(HBpz(3))(2)](+) ({Ru-2(III)(mu-OH)}), was prepared by an in situ conversion of its mu-oxo precursor, [Ru-2(III)(mu-O)(mu-CH3COO)(2)(HBpz(3))(2)] ({Ru-2(III)(mu-O)}), in a Nafion membrane matrix, where HBpz(3) is hydrotris(1-pyrazolyl)borate. The conversion procedure results in fine nanoparticle aggregates of the {Ru-2(III)(mu-OH)} units in the Nafion membrane (Nf-{Ru-2(III)(mu-OH)}), where an average particle size (4.1 +/- 2.3 nm) is close to the Nafion's cluster dimension of similar to 4 nm. Chemically modified electrodes by using the Nafion molecular membrane films (Nf-{Ru-2(III)(mu-OH)}-MMFEs) were further developed on ITO/glass and glassy carbon electrode (GCE) surfaces, and a selective reduction of nitrosonium ion (NO+), presumably through reaction of a {(RuRuIII)-Ru-II(mu-OH)} mixed-valence state with HNO2, was demonstrated without interference by molecular oxygen in an acidic aqueous solution. The Nf-{Ru-2(III)(mu-OH)}-MMFEs are stable even in a physiological condition (pH 7), where the naked {Ru-2(III)(mu-OH)} complex is readily transformed into its deprotonated {Ru-2(III)(mu-O)} form, demonstrating an unusual stabilizing effects for the {Ru-2(III)(mu-OH)} unit by the Nafion cluster environment.