The results of a calculation of the benzimidazolium cation and its 5(6)-substituted derivatives by the Hückel MO method show that the ±I effects of substituents have a considerably lesser influence on the magnitude of the charge on the C₍₂₎ atom than ±M effects. Replacement of C₍₅₎ by a positively charged nitrogen atom (imidazo[4,5-c]pyridinium dication) increases the charge on C₍₂₎ to the same extent as the introduction of a substituent with a vacant p_Z orbital into the 5 position. In accordance with this, the methyl group in the 2 position of the 1,2,3,5-tetramethylimidazolio[4,5-c]pyridinium ion condenses with salicylaldehyde, but the merocyanine cannot be isolated from the 1,3,5-trimethyl-2-(o-hydroxystyryl) imidazolio[4,5-c]pyridinium diperchlorate because of the irreversible degradation of the latter.