Data arc presented showing the changes in physical properties which occur during the calcination of Bayer-process gibbsite at temperatures up to 1400°C. The various stages of the calcination were examined using differential thermal analysis, X-ray diffraction, low-temperature nitrogen adsorption, optical and electron microscopy, and particle-size distribution analyses. The morphology of the gibbsite aggregates was found to be unchanged up to 1100°C, despite the development of several intermediate metastable phases. At higher temperatures, the original gibbsite pseudomorphs become microporous coincident with the development of finely divided alpha-alumina which is formed in a strain-free state. The crystal size and pore size increase with temperature between 1100 and 1400°C. The influence of the parent gibbsite structure on the morphology of the alpha-phase material is demonstrated in scanning-electron micrographs. The effect of various parameters on the milling of high-temperature calcines is also discussed