Thermal transformations of synthetic pyrophyllite prepared by hydrothermal reaction of kaolinite and silica were investigated, using infrared spectroscopy, X-ray diffraction and thermal analysis. It has been found that an intermediate phase, i.e. dehydroxylate of pyrophyllite, is formed as a result of heating pyrophyllite within the range 500 to 800 °C. A dehydroxylate of pyrophyllite is generally of the same structure as pyrophyllite. Decomposition of dehydroxylate into mullite and cristobalite starts at about 1000 °C. The thermal transformations of synthetic pyrophyllite are compared with the thermal stability of natural pyrophyllite from South Africa.