The cubic-to-tetragonal phase transition in the mixed perovskite K 1؊x Li x MnF 3 (x < 0.036) was investigated by single-crystal X-ray diffraction. Results of the structure refinement at 298 and 145 K are discussed. They confirm the cubic symmetry, the space group Pm3m, of the room temperature phase with strongly anisotropic thermal vibration amplitudes of the F atoms, which were also observed in the parent KMnF 3 . Substitutional lattice imperfections at the K ؉ sites shift the ferrodistortive phase transition to higher temperature T c1 ؍ 191 K, compared to 186.5 K in KMnF 3 . The structural changes in the tetragonal phase (I4/mcm) are characterized by the displacements of the F atoms from the positions they occupy above T c1 . However, the Li ؉ ion dopants do not affect the octahedral network, the MnF 6 octahedra remain regular, and no displacements of K ؉ / Li ؉ cations along the tetragonal c axis are observed. The thermodynamic character of the transition is illustrated with the critical behavior of the unit cell parameters and the temperature-dependent evolution of the tetragonal deformation and the macroscopic strain. A variation of the integrated intensity of superstructure reflections close to T c1 has been assumed to describe the behavior of the order parameter. As far as the precision of the present experiment allows, all these observations show that 3.6% Li ؉ substitution at the K ؉ site does not radically change the nature of the transition in KMnF 3 at T c1 .