The products of hydrothermal reactions of kaolinite at 300 °C and 1000 bars were studied in KOH solutions covering an OH⁻ concentration, [OH⁻], of 1 M to 3.5 M. XRD patterns indicated a notable influence of the [OH⁻] on the reaction. At [OH] ≥ 3 M, the only stable phase was muscovite/illite. The content of muscovite/illite was calculated from the analysis of the diagnostic 060 reflections of kaolinite and muscovite/illite. The results showed a linear dependence of kaolinite and muscovite/illite contents with [OH⁻]. ²⁷Al MAS NMR spectroscopy revealed the formation of small nuclei of K-F zeolite at high [OH⁻]. Finally, modelling of the ²⁹Si MAS NMR spectra indicated that the Si/Al ratio of the muscovite/illite formed was very close to that of muscovite, at least in the mineral formed at low [OH⁻]. In good agreement with the XRD data, the quantification of the reaction products by ²⁹Si MAS NMR indicated a linear decrease of the kaolinite content with increasing OH⁻ concentration.