The fcc-to-rhombohedral phase transformation in Ln6WO12 (Ln = Y, Ho, Er, Yb) was investigated by in situ high-temperature X-ray diffraction. The transformation was determined to be an irreversible, first-order, diffusional ordering process, which occurred over a temperature range starting at 1240°C. The transformation can be described as a shape change in the rhombohedral unit cell in the parent fcc phase, determined as elongation along the  axis and contraction in the (111) plane, resulting in an overall volume decrease on heating. A discontinuity in both volume per formula unit and rhombohedral angle α was observed across the transformation region for all four compositions. Trends in lattice parameters, ΔV and Δα at room and transformation temperatures were established according to Ln cation type and were found to closely mirror the cationic radii sequence of the corresponding rare earth elements.