[en] The preparation and properties of N³, O⁴-ethylene-1-methyluracilium methanesulfonate, a heteronuclear stabilized cation, and its interconversions with 3-(β-methanesulfonyloxyethyl)-1-methyluracil were studied. The former was shown to have three sites for reactions with nucleophilic reagents: the β carbon of the ethylene moiety and C-4 and C-6 of the pyrimidine ring. Products resulting from attack at the β position were observed with DMSO, water, alcohols, benzoate, chloride, diethylamine, and pyridine. A strong rate dependence on solvent was noted with chloride ions. Products resulting from attack at C-4 were observed with water, hydroxide, alcohols, alkoxide, and isopropylamine. Diethylamine was the only reagent which led to a product resulting from attack at C-6 of the cation. Oxygen-18 experiments verified the sites at which the uracilium salt reacted with hydroxide and water. Although the N³, O⁴-ethylene-1-methyluracilium cation bears a net positive charge, deuterium exchange reactions were not observed. Mechanisms are proposed to account for the products of the various reactions which were investigated. (U.S.)