The structure of N-methylpyrido[2,3-b]pyrazinium salts has been examined by 1H and 13C NMR. 6-Dimethylamino- and 6-morpholino-pyrido[2,3-b]pyrazines were shown to undergo quaternization by methyl iodide at N-4 of the pyrazine ring, whereas the pyridine ring N-5 was the N-alkylation site in the reaction of unsubstituted pyrido[2,3-b]pyrazine with methyl iodide under the same conditions. The effects of quaternization on the 1H and 13C chemical shifts and the nJ(CH) values are discussed.