The evolution of a Fe/z′-TiOx/Pt(111) model catalyst in ultrahigh vacuum, studied by advanced surface science tools, reveals various phenomena occurring in different temperature ranges. In the room temperature (RT) < T < 460 K range, the deposited Fe atoms assemble as heterogeneous Fe and FeOx nanoparticles (NPs), templated by the ordered array of defects (troughs and picoholes) of the oxide film. At 460 < T < 810 K, FeOx NPs become predominant, and a metastable FeOx/TiOx mixed oxide is formed, assisted by the interdiffusion of Fe through the oxide into the Pt substrate, which triggers the z′-TiOx structure transformation into a different TiOx phase with a hexagonal pattern. At 810 < T < 1000 K, a Fe-mediated extraction of Pt produces Pt(111) islands, surrounded and partly encapsulated by the same z′-TiOx phase, demonstrating the intrinsic high stability of the ultrathin oxide film. This represents one of the first examples of a phase transformation in a ultrathin oxide layer induced by an external metal deposit.