By means of UV absorption and fluorescence spectroscopy, we found that the ground- and excited-state behavior of the monoanion of 2-(6 ' -hydroxy-2 ' -pyridyl)benzimidazole (1) is solvent dependent. In basified protic solvents (water, ethanol), a tautomeric equilibrium exists in the ground state between the monoanion deprotonated at the hydroxyl group (normal anion) and its keto tautomer, resulting from a proton transfer from the benzimidazole N-H to the pyridine N (keto anion). In basified aprotic solvents (acetonitrile, DMSO), only the keto anion is present in the ground state. In protic solvents, the normal anion experiences in the excited state a solvent-catalyzed proton transfer (more efficient in ethanol than in water) to give the keto tautomer. The phototautomerization is temperature dependent in ethanol, the process not taking place at low temperature. We propose that in the ground state, most of the normal anion molecules are hydrogen-bonded to ethanol in such a way that the structure of the solvate is inappropriate to tautomerize. Upon excitation of the normal anion, a two-step tautomerization occurs. The first step involves a solvent rearrangement to form a cyclical solvate with the normal anion, and the second step implies a very fast multiprotonic transfer within the solvate to give the keto anion.