Abstract The frequencies of the transverse and longitudinal modes of the i.r. active E u species of CdCl 2, CdBr 2 and CdI 2 crystals were obtained from the measurements of the far-i.r. reflection spectra. The normal coordinate analysis of the optically active vibrations was carried out on the basis of the rigid ion model. The obtained effective ionic charges of halogen ions are −0.718e, −0.694e and −0.667e for Cl, Br and I, respectively. The short-range repulsive force constants for Cd- X ( X = Cl, Br, and I) are 0.540, 0.427 and 0.277 mdyn/Å, respectively, indicating that the covalent character is not significant for these crystals.