Abstract Hydrothermal synthesis of kaolinite using amorphous alumina and silica as a starting material has been studied in the temperature range from 250 to 350°C and the corresponding saturated water vapor pressure. The pH varied from 1 to 5, the time of treatment from 3 to 70 days, and the percentage of autoclave water from 36% to 72%. The aim of the work is to demonstrate the factors influencing synthesis and the conditions under which the best crystallized product can be obtained. It was found that pH 3, 300°C, and the time interval starting with 5 days produced the best ordered crystals. At higher temperatures (up to 350°C) pyrophyllite was formed along with kaolinite. At lower temperature or shorter time of treatment, poorly crystallized products appeared. The lowering of pH to the value of 1 and 2 with sulphuric acid resulted in formation of better crystallized kaolinite, but alunite was formed at the same time.