The determination of the atomic-scale structure of a solid–solid interface is a major outstanding problem in the physical sciences, the structure controlling many properties including stability, ionic and electronic transport, magnetism, multiferroicity and superconductivity. NMR spectroscopy is sensitive to local structure but is not typically sufficiently sensitive or selective to observe solid–solid interfaces. In this work, CeO₂–SrTiO₃ vertically aligned nanocomposite (VAN) thin films are studied and, by combining selective isotopic enrichment with a lift-off technique to remove the substrate, the ¹⁷O NMR signal from single atomic layer interfaces can clearly be seen. The interfacial structure is solved by calculating the NMR parameters using density functional theory combined with random structure searching. By performing the isotopic enrichment at variable temperatures, the superior oxide-ion conductivity of the VAN films compared to the bulk materials is shown to arise in part from enhanced oxygen mobility at this interface; oxygen motion at the interface is further identified from ¹⁷O relaxometry experiments. These results highlight the information that can be obtained on interfacial structure and dynamics with solid-state NMR spectroscopy, in this and other nanostructured systems, our methodology being generally applicable to overcome sensitivity limitations in thin-film studies.