Crystalline Cs 2 KMnF 6 , when prepared below 5003C, adopts a tetragonal elpasolite structure type. Di4erential scanning calorimetric investigations indicated that Cs 2 KMnF 6 undergoes a phase transition from the low-temperature tetragonal phase (LT) to a high-temperature phase (HT) at about 5303C. Single crystals of the new HT phase could be obtained by annealing a crystalline LT specimen at 6003C followed by rapid quenching to room temperature. In the present study the structures of both phases have been studied by single-crystal X-ray di4raction techniques. The LT phase has the tetragonal space group symmetry I4/mmm, with unit-cell parameters a ؍ 6.319(1) (a ' (2 ؍ 8.936) and c ؍ 9.257(2) A s , and Z ؍ 2. The HT phase has the cubic symmetry Fm3m, with the cell parameter a ؍ 9.067 A s and Z ؍ 4. Structural models of the LT and HT phases have been re5ned vs collected single-crystal X-ray re6ection data to R values of 0.034 and 0.022, respectively. The uneven Mn+F bond distance distribution in the LT form, four bonds of 1.860(6) two of 2.034(9) A s , are typical for an octahedrally coordinated highspin Mn 3؉ ion a4ected by Jahn+Teller e4ects. Due to symmetry constraints, all six octahedral Mn+F bonds in the HT form are equal to 1.931(5) A s . However, the mean square atomic displacement parameters of the 6uorine atoms increases signi5cantly from about 0.022 A s 2 for the LT phase to 0.042 A s 2 for the HT phase. The increased displacement parameters indicate that the phase transition from the LT to the HT form is associated with a directional disorder of the Jahn+Teller distortions around the Mn 3؉ ions.