Reactions of methylsulphinyl derivatives of pyridine, pyrazine, pyridazine, quinoline, isoquinoline, quinoxaline, phthalazine, and cinnoline towards oxidation, reduction (by hydrogen iodide), and nucleophilic replacement (by aqueous sodium hydroxide, n-butylamine, and hydrazine) are described. Sulphoxides with aqueous sodium hydroxide at elevated temperatures gave not only the expected hydroxy-analogue but also a significant amount of the corresponding sulphide.