Methylation and benzylation of 5 (or 6)-nitro benzimidazoles have been carried out under, uniform conditions and the structures of the products obtained have been established by comparison with authentic samples prepared by standard methods. In each case a mixture of 1, 5- and 1, 6-isomers has been obtained, the former being comparatively in larger proportion. The results are explained on the basis of tautomer stabilisation by and the deactivating influence of nitro group.