The excited-state acid-base behavior of 2-(4’-pyridyl)benzimidazole in aqueous solution has been studied over a wide range of acidity by UV absorption and fluorescence spectroscopy. The species detected in the lowest electronically excited singlet state turned out to be identical to those of the ground state : the anion A, the neutral molecule N, the monocation C protonated at the benzimidazole N3, the monocation T protonated at the pyridyl nitrogen, and the dication D. Nevertheless, the strong increase in the basicity of the pyridyl nitrogen atom in the excited state induces different excited-state proton-transfer processes. The dual fluorescence of 4PBI at neutral pH is explained by the protonation of N* to give T*. On the basis of the steady-state data together with the fluorescence decay measurements at several acidities, a mechanism is proposed to explain the acid-base behavior of 4PBI in the first excited singlet state.