A new compound consisting of TDS (thiamine disulfide) and octadecanoic acid was formed when TDS was dissolved in melted octadecanoic acid and the mixture was kept standing to solidify. This compound was crystalyzed by keeping a warm solution of TDS and octadecanoic acid in 1,2-dichloroethane standing. Additional six compounds composed of TDS and higher saturated fatty acids containing twelve to seventeen carbon atoms were prepared in a similar manner. The determination of TDS and fatty acid in these compounds was carried out and their physicochemical properties were investigated by means of differential scanning calorimetry, infrared spectroscopy and powder X-ray diffractometry. Consequently, it was concluded that TDS and fatty acid formed the complex in solid state and the molar ratio of TDS to fatty acid was 1 : 6 in each complex.