3,4-Dihydropyrimidin-2(1H)-ones were efficiently converted into the corresponding pyrimidin-2(1H)-ones in high yields within a short period of time on treatment with aqueous acetonitrile using 1,4-bis(triphenylphosphonium)-2-butene peroxodisulfate. Chemoselective oxidation of 3,4-dihydropyrimidin in the presence of other oxidizable functional groups was also achieved by this reagent.