Changes of stoichiometry in reducible oxides are inevitably accompanied by changes of the oxide structure. We study the relationship between the stoichiometry and the structure in thin epitaxial films of reduced ceria, CeOx, 1.5 ≤ x ≤ 2, prepared via an interface reaction between a thin ceria film on Cu(111) and a Ce metal deposit. We show that the transition between the limiting stoichiometries CeO2 and Ce2O3 is realized by equilibration of mobile oxygen vacancies near the surface of the film, while the fluorite lattice of cerium atoms remains unchanged during the process. We identify two surface reconstructions representing distinct oxygen vacancy ordering during the transition, a (√7 × √7)R19.1° reconstruction representing a bulk termination of the ι-Ce7O12 and a (3 × 3) reconstruction representing a bulk termination of CeO1.67. Due to the special property to yield ordered phases of reduced ceria the interface reaction between Ce and thin film ceria represents a unique tool for oxygen vacancy engineering. The perspective applications include advanced model catalyst studies with both the concentration and the coordination of oxygen vacancies precisely under control.