Nature Catalysis, Published online: 26 June 2026; doi:10.1038/s41929-026-01561-4 Surface composition governs the catalytic performance of any given material but in mixed oxides it can be particularly challenging to determine this with atomic precision. Now by means of in situ layer-by-layer titration it is shown that single-layer SrO termination maximizes oxygen incorporation on (La0.5Sr0.5)FeO3−δ perovskite.