The object of this work is to study the physicochemical proprieties and the thermal behaviour of two Tunisians kaolins. The two samples selected from Tabarka and Kasserine are tested by chemical analysis (CA), X-ray diffraction (XRD), differential thermal analysis (DTA), infrared spectroscopy (IR), cation exchange capacities (CEC) and specific areas (BET) in order to determine their chemical and mineralogical composition and structural differences. This study showed that the clay of Tabarka is a well crystallized kaolinite and the clay of Kasserine is a halloysite 4H2Ò, containing amorphous alumina. The evolution of specific surface according to the temperature of the Kasserine samples after thermal treatment shows an abrupt diminution at 700°C that are interpreted by a process of sintering. The thermal treatment of the two clays at 1200°C, shows the apparition of a new phase that is the mullite. This mullite is identical to mullite gotten by the process sol-gel from the synthesis of the pure aluminosiloxane.