Real-time attenuated total-reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), in conjunction to cyclic voltammetry, was applied to investigate the adsorption and reduction of nitric oxide at Ru electrode in acidic solutions. For a NO-predosed Ru electrode, only one band located at 1840-18,74 cm(-1) was detected in 0.1 M HClO4, attributable to atop NO coadsorbed with oxygen-containing species [designated upsilon(2)(O)-NO species]. For a Ru electrode in 0.1 M HClO4 containing 20 mM NaNO2, two IR bands were observed at 1850-1886 cm(-1) and 1.740-1790 cm(-1). The former, predominant at relatively high potentials, is ascribable to the upsilon(2)(O)-NO species; the latter, to atop NO adsorbed on nominal Ru sites at relatively low potentials (designated upsilon(2)-NO species). In addition, a very weak band at 1520-1578 cm(-1) may be assigned to multicoordinated NO coadsorbed with oxygen-containing species. The real-time spectral results suggest that the reduction of NO molecules and the coadsorbed oxygen-containing species proceed simultaneously rather than separately. (C) 2007 Elsevier Inc. All rights reserved.