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 degrees 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 [111] 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 was observed across the transformation region for all four compositions. Trends in lattice parameters, V and 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.