The crystal structure of the polymorphic form of thiamine iodide hydro iodide has been determined using single crystal X ray diffraction method. The crystal data obtained are: space group: P21/n, a=13.7883(10)Å, b=7.4330(5)Å, c=17.8082(13)Å, β=101.621(2)o Z=4. The cations and anions are linked by O-H--I hydrogen bonds, CH-- I interactions and I-- thiazolium ring close contacts. The thiaminium cation adopts a F conformation. FT-IR and CHNS studies supports the crystal structure and the molecular formula. TG/DT analysis was carried out to study its thermal properties. UV-Vis -NIR spectroscopy and dielectric studies at ambient temperature were used to determine the energy gap (Eg) and polarisability of the grown crystals.