DHB_1 (Dihydrothiamine) was oxidized almost quantitatively into thiamine, when its buffered acetic solution (pH 4) was kept at 40℃ for 10 minutes with the 50mg% solution of 2,6-dichlorophenolindophenol (I). Thus formed thiamine was adsorbed on a zeolite-column and was easily estimated by thiochrome method after its elution. Nearly quantitative reduction of (I) was also observed and the yield of thiamine from pseudo-DHB_1 was 90〜100% to the theoretical value, when 20-fold molecular equivalents of (I) were given to DHB_1. According to this procedure proposed by the authors, oxidation into thiamine was completed within 5 minutes, yielding the better results than the FeCl_3-oxidation method.