The aim of this work was to understand the nature of high conductivity of the perovskite fluoride KCaF3. A series of experiments were carried out using single crystal neutron diffraction. Fourier maps reveal a high degree of anisotropy in the F- ion vibrations. Comparison is made between harmonic and anharmonic models for thermal vibration. The results show that KCaF3 is a straightforward structure with a low concentration of defects. There is no significant occupation of interstitial sites and it is likely that the high conductivity is due to a small population of highly mobile vacancies. A model for the conduction mechanism is discussed. (c) 2005 Elsevier B.V. All rights reserved.