The hydrothermal reaction of kaolinite in the system Na2O-K2O-(MgO)-Al2O3-SiO2-H2O has been investigated at 200 and 300°C. The study of the solid products indicates that, at 200°C, disordered illite was the first phase formed, which evolved at increasing run times towards well ordered illite and analcime. After longer reaction times illite and kaolinite were the sole phases identified by XRD. The b parameter of the illites increased at longer reaction times, from 9.05 to 9.08 Å. Study by TEM/AEM revealed the presence of surface chlorite-like layers, covering the illite packets, however. The trend observed in the products of the reaction at 300°C was similar, except for the presence of boehmite or pseudoboehmite at the higher temperature. The values of the b parameter of illite, of the order of 9.12–9.16 Å, indicate that illites formed at 300°C have a more trioctahedral character. The results obtained also indicate that Mg determines which product will be formed, K-and Nazeolites and Al-rich illites being the phases formed in the absence of Mg.