The tautomeric forms of benzimidazole N-oxide derivatives in solution were studied using nuclear magnetic resonance (NMR) techniques. Further insight into the molecular structures was provided by theoretical calculations using density functional theory (DFT). In the gas phase the N-hydroxy tautomer was more stable than the N-oxide, whereas in solution the stabilization of one form or the other depended on hydrogen bond formation involving the N-hydroxy/N-oxide moiety. Derivative 4 (n-butyl-5-nitrobenzimidazole-2-carboxamide 3-oxide), having a 2-carboxamide moiety, was the only compound studied present as a mixture of tautomers, the N-oxide being the predominant one. This was assigned to the formation of an internal hydrogen bond between the N-oxide group and the amide hydrogen atom. The tautomeric form present in the solid state was studied for derivative 1 (ethyl-5-nitrobenzimidazole-2-carboxylate3-oxide) and was conclusively assigned by X-ray diffraction techniques to the N-hydroxy tautomer. In the crystal a strong O-(HN)-N-... intermolecular bond gives rise to supramolecular polymeric chains in the lattice. This strong interaction was also seen in the infrared spectrum and was assigned to two broad bands at 2367 and 2526 cm(-1). The vibrational spectrum was satisfactorily described by DFT calculations and an example of this is the prediction of the band corresponding to the N-O stretching (N-oxide) just 1% lower than the experimental value. Uncorrelated calculations (HF) were not able to give an unambiguous assignment of this band. The reaction of derivative 1 against different kinds of electrophiles, hard and soft, led only to O-substituted products. This result was explained in terms of the HSAB theory using a local-global approach.