Infrared reflection spectra at room temperature have been investigated for TGS-family ferroelectric crystals, including TGS. DTGS and ATGSP. The observed lines are tabled and classified according to their wavenumber and modes. We have found that there is a line at 930cm-1 resnlting from PO43--ions group on the infrared spectra of ATGSP. This indicates that the phosphoric acid enters the crystal, replacing sulphuric acid.