The interactions between O-benzoylthiamine disulfide (BTDS) or its related compounds and proteins were studied by the methods of paper chromatography and zone electrophoresis. Thiamine-protein complexes and free thiamines were produced by the reaction of BTDS or other symmetrical thiamine disulfides with denatured egg albumin or with serum albumin, but these complexes were not formed by the reaction with the denatured proteins with blocked SH groups, as well as, with native egg albumin. The complexes were not separated to their components by means of paper chromatography or zone electrophoresis, but thiamine was liberated by the reaction with sodium thiosulfate. Therefore, the complex formation was supposed to be concerned with the SH group of proteins and seemed to be caused by the disulfide interchange between the SH groups of proteins and the S-S groups of the thiamine derivatives.