Comparative studies of the electrochemical reduction of 2-(o-nitrophenylthio)-acetic acid and its methyl ester in aqueous media of different acidity show that reduction of the ester in an ammoniacal buffer is the best adapted for electrosynthesis of 4-hydroxy-2H-1,4-benzothiazin-3(4H)-one. The electrochemical behaviours of the corresponding phenylhydroxylamines and azoxy compounds are also investigated.