The I-II phase transition occurring in KSbF 6 at 302 K has been investigated by means of infrared spectroscopy. The majority of translational and rotational modes observed below 150 cm - 1 could be assigned in both phases. The phase transition is of the first-order, and a librational mode, representing rotations of the anions around the C 3 -axis in the cubic phase II, shows soft-mode behaviour when the transition temperature is approached from lower temperatures. A discrete symmetry is broken when the low-temperature cubic phase II is transformed into the tetragonal phase I at 302 K. A comparison of the calculated and observed values of the II-I transition entropy in KSbF 6 shows that although they are of the same order of magnitude, the reorientational motions of the anions are probably more complex than those represented by 4-fold reorientations around the S 4 -axis of the space group P42m in phase I, and 3-fold reorientations around the C 3 -axis of the space group I2 1 3 in phase II.