Capturing OH(H ₃ O ⁺ ) in ionized water Recent advances in liquid-phase ultrafast electron diffraction techniques make it possible to observe what has only been theoretically presumed to occur at short times upon interaction of ionizing radiation with liquid water. Lin et al . provide direct evidence for capturing the short-lived radical-cation pair OH(H ₃ O ⁺ ), which has been hypothesized for years to form after ionization of liquid water but was not structurally resolved previously (see the Perspective by Cao et al .). The authors trace dissociation and subsequent nonradiative structural relaxation around the ionization center. —YS