Acid-base behaviour of some 2-(n'-pyridyl) benzimidazoles (n' = 2,3,4) have been studied in order to search the protonation site and to predict pK(a) values of these molecules both in gas and aqueous phases using semi-empirical quantum chemical calculation methods. The best correlation between the experimental acidity constants, pK(a), and proton affinities was detected with PM3 method in aqueous phase. Furthermore, acceptable correlations were observed between the experimental acidity constants, pK(a) (exp.) and calculated acidity constant, pK(a) (calc.) by AM1 and PM3 methods. A change-over in the protonation mechanism for the second proton-gain process for all compounds was observed. (C) 2002 Elsevier Science B.V. All rights reserved.