Abstract This paper reports an investigation on the effect of mineralogy on the pozzolanic activity of fired kaolin. Representative samples of Greek kaolin (Milos Island) and a commercial kaolin of high purity are studied. The samples are tested by X-ray diffraction (XRD), Differential Thermal Analysis (DTA) and Infrared (IR) Spectroscopy in order to determine their mineralogical composition and structural differences. Calcination of samples is carried out at 650°C for 3 h. The decomposition of kaolinite and alunite is recorded using methods of thermal analysis. The resultant products are identified by XRD. The reactivity of the thermally treated samples is evaluated based on Chapelle test. It is concluded that the pozzolanic activity of metakaolinite is strongly related to the crystallinity of the original kaolinite. Well-ordered kaolinite is transformed into less reactive metakaolinite.