KY₃F₁₀ (Fm¯3m, Z = 8) is an anion-excess fluorite-related superstructure, which is employed as a room-temperature laser, when doped with rare-earths. Earlier conductivity measurements have revealed two thermally activated processes below and above 600 K. In this work we studied the high-temperature behaviour of KY₃F₁₀ in the temperature range from 293 K up to 800 Kat ambient pressure using x-ray powder and single crystal diffraction. No structural phase transition is observed in this temperature range and the structure determinations reveal that the change of structural parameters, interatomicdistances, and angles with temperature is very small. A prominent maximum observed in the difference Fourier maps, which corresponds to the centre of an empty F₈ cube, might be one of the interstitial sites involved in the conduction mechanism.