Using “time-resolved X-ray absorption spectroscopy” (XAS), which can capture detailed information about the electronic structure of specific atoms in a molecular system, as well as the geometry around them, Majed Chergui and co-workers were able to confirm for the first time that an intermediate, “domed” structure, does indeed occur after nitric oxide rebinds to heme, unlike with any of the other diatomic ligands (cyanide, carbon monoxide etc). They also showed that the 200-picosecond timescale is actually due to nitric oxide molecules coming from remote docking sites in the protein. This work opens the way to a detailed investigation of metalloproteins using subpicosecond X-ray spectroscopy at free electron lasers.