Application of alkaline benzenesulfonyl chloride to thiamine results in the formation of thiamine anhydride (IV), but it had been surmised that this reaction proceeds through intermediate formation of O, O′-bis (benzenesulfonyl) thiamine disulfide (II) or O-benzenesulfonylthiamine (III). In order to compare the change of (II) and (III) to (IV) in alkaline medium quantitatively, N sodium hydroxide solution was added to 1-100mg% solution of (II) and (III), and residual amount was determined (Table I). (III) almost completely disappeared on being made alkaline and formation of (IV) was detected by paper chromatography. (II) changed only slightly in alkaline medium and 15-35% remained 60 minutes later. (III) underwent decomposition at above pH 8, while (II) decomposed above pH 11 (Table II). These results suggest that (III) is produced as an intermediate for the formation of (IV) from thiamine and that (III) undergoes cyclization in alkaline medium.