Transmission spectrum ofCsDy(MoO4)2single crystals is measured in polarized light in the longwave IR region (10–100 cm−1)at low temperatures 6–40 K. The change in the vibrational spectrum during first-order structural phase transition(Tc∼38 K)is detected experimentally. The vibrational spectrum ofCsDy(MoO4)2is analyzed in the model of “quasi-chipped off” vibrational modes. ©1998 American Institute of Physics.