The work is aimed at studying of transient processes in thermosyphons. The calculated dependences for determining the basic temperature characteristics of the controlled object are obtained. For this purpose, the thermal resistances of main elements in a thermosyphon have been wrote and calculated. The equations for calculating the change in saturation temperature in a thermosyphon are proposed. These changing strongly depends on the design and operating parameters of the heat exchanger. In this case a thermosyphon is considered as an object of regulation. Gases flow rate changing (or heat exchanger switch off) is considered as an inlet distribution. The time constant expressions and the gain factor with a stepwise increase in the temperature of the gases are obtained. The calculations of the change in the saturation temperature in a thermosyphon with a linear decrease in the temperature of gases are shown. The data of experimental studies are given when the heat exchanger is stopped. For these investigations the changes in the temperature of gases and the saturation temperature in thermosyphons were held. A conducted comparison of the calculated and experimental data shows a good agreement.