Both kaolinite and its thermal decomposition products are to be regarded as “active” solids in that they have a fairly large specific surface area, of the order of 10 square metres per gram. It was therefore thought worth while to apply some of the procedures which proved useful in the investigation of other surface active solids, to the problem of the dehydration of kaolinite. The techniques chosen were: (a) the resorption of water by the dehydration product; (b) the sorption isotherm of a vapour (carbon tetrachloride) on the dehydration product, in order to estimate its surface area; (c) the apparent density of the product in a suitable, inert, immersing liquid; (d) X-ray examination of the product (through the kindness of Dr Clark and Mr Parker of English Clays Lovering Pochin Ltd.).