X-ray photoelectron spectroscopy (XPS) (with AlKα and AgLα radiations) and scanning tunneling microscopy (STM) were used to study the interaction of two model samples prepared by vacuum evaporation of platinum on highly oriented pyrolytic graphite (HOPG) with NO2 at room temperature. According to STM data, platinum evaporation on the graphite surface produced particles of a flattened shape. In the Pt/HOPGS1 sample with a lower concentration of platinum, the average diameter of particles d and the height-to-diameter ratio h/d were 2.8 nm and 0.29, respectively. In the Pt/HOPG-S2 sample with a higher concentration of platinum, the average values of d and h/d were 5.1 nm and 0.32. When the samples interacted with NO2 (P ≈ 3 × 10–6 mbar), the particles of metallic platinum completely converted to the particles of PtO2· Upon oxidation, the shape of larger platinum particles in the Pt/HOPG-S2 sample did not change, although for the dispersed particles in the Pt/HOPG-S1 samples under these conditions, the h/d ratio increases. The reduction of oxide to metal particles on heating the Pt/HOPG-S1 sample in vacuum at 460°С is accompanied by an increase in the size of particles. Their shape became more round compared to the initial one. It was found that X-ray radiation affects the state of platinum in the oxidized sample by reducing the surface layer of PtO2 to PtO.