The ground- and excited-state tautomerism of 2-(6'-hydroxy-2'-pyridyl)benzimidazole (1) and 1-methyl-2(6'-hydroxy-2'-pyridyl)benzimidazole (2) in various solvents was investigated by means of UV-vis absorption spectroscopy, steady-state and time-resolved fluorescence spectroscopy, and quantum-mechanical ab initio calculations. A solvent-dependent tautomeric equilibrium was observed for both compounds in the ground state between the lactim or normal form and the lactam tautomer resulting from a proton translocation between the hydroxyl group and the pyridine nitrogen. Here, we report evidences for a solvent-dependent switching in the nature of the excited-state proton transfer (ESPT) reactions undergone by 1 and 2. In the aprotic solvent acetonitrile, no significant ESPT takes place. The protic solvents ethanol and water facilitate the proton transfer through bridges of solvent molecules, but each solvent catalyzes specifically a different ESPT process. In aqueous solution, the excited lactim species of 1 and 2 undergo a proton transfer from the hydroxyl group to the pyridine nitrogen, favoring the lactam tautomer in the first excited singlet state. In ethanol the ESPT takes place for 1 from the benzimidazole NH to the pyridine nitrogen, originating a new tautomer not observed in the ground state. Compound 2, without a NH group, does not tautomerize in the excited state in ethanol.