Investigations of magnetic and spontaneous birefringence properties of aKDy(MoO4)2crystal under the Jahn–Teller phase transformation reveal some features indicating the existence of two spontaneous phase transitions at temperatures close to 14.3 and11.0&hthinsp;K. The characteristic changes in birefringence in these transitions suggest that the crystal of KDy molybdate has a modulated incommensurate lattice structure in the temperature range from 11.0 to14.3&hthinsp;K. ©1998 American Institute of Physics.