NO (nitrogen monoxide) adsorption on a Pt(111) electrode surface yielded an STM image of 2 x 2-Pt(111)-NO structure. At a negative potential where no NO oxidation occurs, weak spots from water molecules were imaged between the bright NO spots. Even at a positive potential where NO oxidation takes place, a similar 2 x 2-Pt(111)-NO2 structure remained. Both images were reproduced reversibly by repeated potential cycles. This means that NO adsorption sites are preserved on the 2 x 2-Pt(111) surface sites during the redox processes (NO reversible arrow NO2). At the point of NO saturation, NO molecules in a 2 x 2 array were adsorbed exclusively on 'atop' sites of Pt(111). However, a new IR absorption band (NO2 band) began to appear and increased its intensity as the NO oxidation proceeded. An adsorbed H2O molecule adjacent to an adsorbed NO served as an oxygen source for the redox process and the oxidation reaction proceeded simultaneously with water decomposition on the surface.