In situ scanning tunneling microscopy (STM) coupled with cyclic voltammetry was used to elucidate electrified interfaces of Pt(111) in solutions of 0.1 M HClO4 containing KNO2. Potential sweeping between 0.5 and 1.1 V generated a pair of redox features at 0.68 and 1.02 V, due to the surface-bound NO/HNO2 redox couple. These features appeared to be structure-sensitive, as they were not found for roughened Pt(111). In situ STM imaging under potentiostatic conditions revealed two ordered adlattices, (2 x 2) and (root 13 x root 13)R14 degree at 0.5 and 1.0 v, probably due to adsorbed NO and HNO2, respectively. The anodic wave at 1.02 V contained ca. 120 muC/cm(2) charges, indicating a coverage of 0.5 (two NO molecules per unit cell) for the (2 x 2) structure, although the study identified three tunneling maxima within each unit cell. This work presents a real-space model, featuring two NO molecules and one OH species coordinated at 3-fold hollow and on-top sites respectively. The (root 13 x root 13)R14 degree structure observed at 1.0 V contains six protrusions per unit cell, corresponding to a coverage of 0.46. The well-defined corrugation pattern in the STM molecular resolution allowed a real-space model of HNO2 molecules. This structure is likely to contain extended intermolecular hydrogen-bonding chains along the root 13 directions. This intermolecular interaction might influence the reversibility of the redox couple of NO/HNO2.