The physico-chemical characterization of three samples of clay used in Nabeul (TUNISIA) ceramic industry are studied by some methods: chemical analysis, X-ray diffraction, infrared spectroscopy, differential thermal and thermogravimetry analysis, dilatometry, measurement of surface areas and cation exchange capacities. The samples are kaolino-illitic clay, but they differ in cristallinity and in the rate of quartz and iron. A quantitative estimation of the ratio kaolinite/illite by three methods (X-ray diffraction, infrared spectroscopy, differential thermal analysis) are proposed.