DL-Homocysteic acid (DLH, (NH3 )-CH(COOH)-(CH2 )2-SO3-) is extensively in existence as free amino-acid in the central nervous system of mammal. The infrared (IR) and Raman spectra of DL-Homocysteic acid (DLH) and its La complex La (DLH)2Cl3 . H2O = LaL2 were measured at high external pressure. The DLH has two pressure-induced phase transitions (near 17 kbar and 37 kbar) and three pressure-phase areas below 50 kbar for there are ten intermolecular hydrogen bonds between the DLH molecules. But there is only one pressure-induced phase transition (near 27 kbar) to be observed in the complex LaL2. These show that the ten intermolecular hydrogen bonds of the DLH molecules changed due to the formation of complex LaL2. In infrared spectra, the changing trend of the pressure sensitivities (dv/dp) of symmetry stretching of SO3- in DLH and asymmetry stretching of SO3- in LaL2 are very different from the other vibrational modes when both compounds undergo high external pressure. In infrared spectra, the average pressur