- Solvation and transport of excess protons in aqueous systems play a fundamental role in acid-base chemistry and biochemical processes. Here, we map ultrafast proton excursions along the proton transfer coordinate by 2D infrared (IR) spectroscopy, both in bulk water and in a Zundel cation (H 5 O 2 ) + motif selectively prepared in acetonitrile. Electric fields from the environment and stochastic hydrogen bond motions induce fluctuations of the proton double-minimum potential. Within the lifetime of a particular hydration geometry, the proton explores a multitude of positions on a sub-100 fs time scale. The proton transfer vibration is strongly damped by its 20 to 40 fs population decay. Our results suggest a central role of Zundel-like geometries in aqueous proton solvation and transport.