A reaction product was isolated in a pure crystalline form after the reaction of rutin upon thiamine for 7 hours at 70℃ in a medium, pH 7.0. The compound melts at 130℃, having an empirical formula of C_<19>H_<26>O_6N_6S_2. It is soluble in water as well as organic solvents, e.g., ethanol, butanol, benzene, and chloroform. This product was proved biologically active as thiamine by the test using Uroloncha striata var. domestica. After oral administration to rats, thiamine was markedly excreted in urine. These findings led to the conclusion that the reaction product belongs to a new disulfide derivative of thiamine-thiol.