We have determined reaction rates and activation energies for the dehydroxylation of kaolinite by flash calcination using two laboratory and two industrial pilot flash calciners under various conditions. The kinetics of dehydroxylation/water loss can be fitted to a 3-dimensional diffusion model, but changes in mechanism occur at high dehydroxylation levels and lead to deviations from that model. The effects of furnace gas and particle size are discussed.