According to the results of the experiment on the factors of reduction or hydrolysis of thiamine disulfides or O, S-acylthiamine and thiaminase activity in the thiaminase solution of fern, shell-fish, Bacillus thiaminolyticus Matsukawa et Misawa and Bacillus aneurinolyticus Kimura et Aoyama, it was recognized that heat stable and enzymatic reductive factors and a hydrolytic factor exist in the thiaminase solutions, and that thiamine disulfides and O, S-acylthiamine are at first step reduced or hydrolysed, then decomposed by thiaminase. In comparison between the decomposition grade of thiamine and that of thiamine disulfieds and O, S-acylthiamine in the thiamine solutions, thiamine disulfides are found to be destroyed with more difficultly. Diacetylthiamine is considerably decomposed by the solution of fern and shell-fish which have thiaminase I, but not by bacterial thiaminase solution containing either thiaminase I or II. However, dibenzoylthiamine is generally hard to be destroyed by these thiaminase solutions.