The detailed mechanism of proton transfer and the individual catalytic roles of H2O, H3O+, and OH- in the excited-state tautomerization of aqueous 3-hydroxyquinoline are determined by measuring fluorescence kinetic constants as functions of wavelength, pH, hydrogen isotope, and temperature. The molecule undergoes tautomerization in a stepwise manner by forming an anionic intermediate and the tautomer on time scales of 600 and 1200 ps, respectively, in the pH region of 7-8. However, the intermediate transforms rapidly into the tautomer in the pH region of 4-6 owing to the migration of the proton concurrently formed with the intermediate. This provides experimental evidence of the idea that the extremely fast migration of protons in water occurs by continuously forming and breaking hydrogen bonds involved in hydrated proton clusters.