Nature Catalysis, Published online: 15 July 2026; doi:10.1038/s41929-026-01582-z Surface strontium (Sr) enrichment in perovskite oxides has long been associated with degraded oxygen electrocatalysis. Now, atomically precise control of SrO coverage under operating conditions shows a volcano-shaped structure–activity relationship, revealing that a single SrO surface layer maximizes oxygen incorporation, with further analysis showing how surface termination influences the mechanism.