Radical-mediated proton transfer enables hydroxyl radical formation in charge-delocalized water.

The Grotthuss mechanism has long defined our understanding of proton transfer in water, where protons migrate through hydrogen bond networks structural diffusion. However, whether radical intermediates participate in this process remains unresolved. Herein, we demonstrate a radical-mediated proton transfer mechanism in acidic aqueous solutions. Using electron paramagnetic resonance spectroscopy, we detect hydroxyl radicals (˙OH) and water radical cations (HO˙), while isotope-labeled high-resolution mass spectrometry confirms the presence of crown ether HO˙ complexes in HCl solutions. Quantum simulations reveal that excess charge is delocalized over neighboring water molecules rather than localized on the hydronium ion (HO), forming positively charged water clusters. We thus reveal that the proton transfer proceeds through ˙OH⋯H···HO intermediates under ambient conditions. Our findings extend beyond the Grotthuss framework, proposing a mechanism driven by hydrogen bond imbalance and charge delocalization that spontaneously generates ˙OH in acidic environments. This work advances the understanding of proton transfer in water and has implications for acid-induced reactions, electrochemical processes, and degradation mechanisms in energy technologies.