Investigation was made on the physical and chemical properties of S-benzoylthiamine O-monophosphate (BTMP), C_<19>H_<23>O_5N_4SP・nH_2O. Four crystalline forms named α, β (n=2,plate), γ (n=1.5,needle), and δ (n=0,prism) were found. The crystals are colorless, odorless and nonhygroscopic. On the basis of infrared spectra and X-ray diffraction patterns of the four crystalline forms, similarity in α and β, and an obvious difference between γ and δ were shown. Ultraviolet absorption of its aqueous solution was studied at various pHs and found that the absorption maxima were shifted with change of pH, and the isosbestic points were 239 mμ and 277 mμ. Dissociation constants obtained by the titration curve are K_<a1> 1.64×10^<-6>, K_<a2> 0.98×10^<-7> and K_b 2.00×10^<-12> and isoelectric point was calculated as pH 4.05. Surface tension of the aqueous solution was measured and it was found that BTMP is more surface active than thiamine hydrochloride. Solubilities in various solvents were measured. Alpha crystal is most readily soluble, and the solubility decreased to the minimum value at pH 4. Rf value is 0.48-0.53,when a developing solvent consisted of acetic acid, butanol and water. is used. Relationship between stability of the aqueous solution of BTMP in a concentration of 5mg/ml and pH was studied by means of paper chromatography and fluorometry, and it was shown that BTMP is stable in acidic enviroments, particulary at pH 4,while unstable in alkaline solution.