Some characteristics of the kaolin→metakaolin reaction have been studied by applying nuclear magnetic resonance, X-ray fluorescence, and acid dissolution techniques to samples at intermediate stages of dehydroxylation. Nuclear magnetic resonance spectra enabled the average distance between hydroxyl groups to be computed, while X-ray fluorescence provided information on changes in the co-ordination number of aluminium and acid dissolution demonstrated the reduction in cohesion between individual layers during dehydroxylation. The last technique appears to be useful in studying the development of new solid phases. The main conclusions are that the reaction proceeds in a stepwise manner by complete removal of constitutional water from octahedral sheets and not by the growth of randomly distributed nuclei, that six-fold coordinated aluminium is gradually transformed into four-fold coordinated aluminium, and that the internal surface becomes progressively accessible to the acid reagent as dehydroxylation proceeds. The ‘ring structure’ of the silica layer remains.