Nature Catalysis, Published online: 13 August 2026; doi:10.1038/s41929-026-01599-4 Understanding nanoscale heterogeneity in electrocatalytic reactions is critical for advancing catalyst design and performance. Now, a single-turnover imaging technique with subparticle resolution, DELINE, is introduced, which enables the mapping of electrocatalytic reactions and is used to probe surface water dissociation.