This work presents a complete surface science investigation of a model system obtained depositing different amounts of Fe on the z′-TiO x/Pt(111) ultrathin (UT) film, whose structure is known in great detail. The system has been investigated both at room temperature (RT) and after thermal treatments in an ultrahigh vacuum at temperatures in the room temperature to 900 K range. In contrast with standard thermodynamic predictions, we show that Fe nanoparticles (NPs) strongly compete with Ti to bind oxygen, i.e., a redox reaction where Fe oxidizes and Ti is further reduced to an extent proportional to the amount of deposited Fe is observed. The z′-TiO x UT film is first destabilized by the presence of Fe, but as soon as the temperature is raised, so activating an interdiffusion of Fe into the Pt substrate, a rather ordered UT TiO x film is formed again. However, a new hexagonal (h-TiO x) phase replaces the z′-TiO x one in the room temperature to 800 K range, which progressively transforms into the most stable z′-TiO x form at the highest temperature (900 K). At the intermediate temperatures, the system is present in the form of FeO x/TiO 2 mixed oxide NPs. This is a paradigmatic example where the nanoscale effects produce unexpected transformations different from those observed in the bulk. © 2011 American Chemical Society.