In order to determine the position of the sulfonic acid group bonded to the thiazole residue, sulfathiazole-NaHSO₃ addition product was methylated with dimethyl sulfate. As the reaction product obtained by its treatment with conc. sulfuric acid happened to be 3-methylthiazolone-acetosulfanilimide, the adition product was proved to be 2-(p-aminobenzenesulfonamido)-thiazole-2-sodium sulfonate. Chemical properties of several sulfonamide-NaHSO₃ addition products were also examined.