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To optimize the catalytic functionality of cerium oxide it is important to understand the structural modifications associated with reduction and the role of the proximity of metals, which are often coupled with the oxide in the applications. For this purpose, the evolution of the short‐ and long‐range structure of cerium oxide ultrathin epitaxial films and nanostructures supported on Pt(111) is investigated using X‐ray absorption spectroscopy at the Ce L3 edge and surface X‐ray diffraction, during reduction by thermal treatments in vacuum. In epitaxial nanoislands reduction is associated with a contraction of the Ce–O distance and with the appearance of CePt bonds. The formation of a phase with a (2 × 2) periodicity after a thermal treatment at 1023 K is ascribed to the formation of a Pt5Ce alloy. Films of 3 nm thickness do not show, on average, significant structural modifications with the same thermal treatment, consistent with the hypothesis that the reduction involves only the topmost surface layers and it does not influence significantly the bulk structure of the material. This study demonstrates a strong interaction between cerium oxide and platinum, which has implications for the reactivity and stability of catalysts based on metals combined with reducible oxides. [ABSTRACT FROM AUTHOR]