The application of the de Saladin-Le Chatelier differential thermal analysis method to the identification of soil clays has given rise to a certain number of difficulties. In fact, it is possible for these clays to be altered either in situ by the action of atmospheric and biochemical agents or in the laboratory during their extraction. Mr Henin and I have been investigating what influence chemical agents can exert on the thermal diagrams of different types of clays. The treatments have been effected by normal solutions of cations in the form of chlorides, carbonates and nitrates, or by N/10 acid or basic solutions.