The reaction products of thiamine by catechol, hydroquinone and rutin, respectively, were isolated in a crystalline form. The three compounds were shown to be identical with thiamine disulfide in respect to ultraviolet and infrared absorption spectra, paper chromatography, elementary analysis and mixed melting point test. The action mechanism of thiaminedecomposing thermostable factor is as follows : Diphenols are oxidized by oxygen to quinones, which in turn oxidize thiamine to thiamine disulfide.