Abstract. The temperature distribution in a thermosyphon was studied experimentally. To conduct the research, a closed two-phase thermosyphon was developed, which differs from the known by simple construction. The method of studying the rapid processes of conduction, convection and phase transitions was also developed. It will allow to highlight the operational modes of the thermosyphon, considering the load, cooling conditions of the condensation section, value of the heat supply. According to obtained results the instabilities of the temperature fields over the crosssection of the two-phase closed thermosyphon were observed by means of using the modern measuring equipment. It has been suggested that the instabilities can be caused by different modes of thermosyphon operation.