Polarized infrared (4000–400 cm −1) and Raman spectra (3600–10 cm −1) of betaine borate single crystal have been recorded at various temperatures. The results presented here show that both phase transitions detected in betaine borate (ferroelastic at 142.5 K and structural at 45 K) manifest themselves in vibrational spectra. The polarized infrared spectra indicated that betaine and boric acid are not planar in ferroelastic phase which is consistent with structural studies. Two optic modes affected by the ferroelastic transition in the lattice vibration region are detected. One of them having B 2g symmetry is coupled linearly with the e 5 component of the elastic stiffness tensor of the same symmetry and is assumed to be involve in ferroelastic transition. This observation points out that betaine borate belongs to the ferroelastic crystal of the optical type. The evidence of the structural transition (at 45 K) were detected in lattice vibration region.