An ore containing kaolinite (Al2O3 . 2SiO(2) . 2H(2)O), alunite (KAl3(SO4)(2)(OH)(6)), feldspar (KAlSi3O8) and quartz (SiO2) was analysed by TG/DTA to determine the mineralogical composition of samples considered representative of concentrates and tailings likely to be obtained from physical treatments designed to produce a kaolin concentrate as low as possible in alunite. The ore was divided into various particle-size fractions each of which was subjected to TG/DTA analysis. A knowledge of the reactions and the decomposition temperatures of the minerals present permits the application of a system of equations that provides the composition of each size fraction, thus circumventing difficulties due to the overlapping of weight-loss curves during execution of the thermal curve. The alunite results provided by TG are in good agreement with those obtained via standard calculation starting from elemental chemical analysis. The kaolinite results are less accurate but as they follow the same trend as those obtained by chemical analyses, except for the finest fractions, they can be used for relative comparison of samples obtained during a physical treatment testing campaign.