In this paper we report the acid–base behaviour of 2-(n′-pyridyl)benzimidazoles (n′ = 2,3,4) and some methyl derivatives in aqueous solution over a wide range of acidity, involving interesting features with respect to competition in protonation between the benzimidazole N3 and the pyridyl N, with the existence of the corresponding two monocations in equilibrium in the case of n′ = 4. The influence of pH on the electronic absorption spectra of all compounds has been studied in order to identify the ground-state species present as a function of acidity and to obtain the acidity constants of the various prototropic equilibria occurring in the wide range of acidity studied. Keywords: 2-pyridylbenzimidazoles, prototropic equilibria, protonation, acidity constants.