The electrochemical properties are explored of hydrous ruthenium oxide adsorbed in form of nanoparticles onto polished boron-doped diamond (BDD) substrates. Hydrous ruthenium oxide nanoparticles are prepared in two different sizes of average diam 8-14 nm (type A) and 1-2 nm (type B), and are shown to readily adsorb in form of a monolayer on polished BDD electrode surfaces. The electrochemically inert character and well-defined electrode surface area of the BDD electrodes allow properties of the nanoparticle layer to be investigated quantitatively and new electrochemical features to be observed. A prominent oxidation peak attributed to the conversion of the hydrous ruthenium (III) oxide to hydrous ruthenium(IV) oxide occurs at 0.82 V vs. saturated calomel electrode at pH 7. The Nernstian pH dependence of the peak potential over a wide pH range (1 to 13) is consistent with an oxidation accompanied by simultaneous expulsion of one equivalent of protons. After cathodic "activation,'' both types of nanoparticulate hydrous ruthenium oxide (type B more efficiently than type A) catalyze the hydrogen evolution reaction. (C) 2002 The Electrochemical Society.