A linear chain model with intralayer force constants has been used to interpret the long wavelength lattice vibrations of the layer structures - CdBr2, CdCl2, MnCl2 and CoCl2. In the nearest neighbour approximation this model gives results in agreement with experimental information for MnCl2 and CoCl2; the discrepancy in the case of CdBr2 and CdCl2 is explained by including the effect of the second nearest neighbour, which would be present because of long range electrostatic forces in these cases. © 1975.