We report and analyze Raman spectra recorded on crystals of LiH 2 PO 4 (LDP) and KLi(H 2 PO 4 ) 2 (KLDP). We provide a new and complete assignment of Raman lines in both crystals from their behavior as function of temperature. Among internal vibrations, we discern the modes within PO 4 tetrahedra, the modes related to Li-O bonds, and those due to the O-H vibrations. In the wavenumber range between 300 and 600 cm −1 , the lines associated with O-P-O or Li-O show different temperature dependencies. The bands of PO 4 are red-shifted with increasing temperature according to thermal dilatation, whereas the positions related to Li-O are temperature independent.