A device for controlling the dehydration operation during a freeze-drying treatment comprises a freeze-drying chamber (1) connected to a vacuum line, and a gas analyzer, for analyzing the gases contained in the chamber. The gas analyzer comprises a gas ionization system (8) comprising a plasma source (13) in contact with the gases, which plasma source is combined with a generator (15) capable of generating a plasma from said gases, and a system for analyzing the ionized gases, comprising a radiation sensor (17) located close to the plasma generation zone and connected to an apparatus (18) for analyzing the change in the radiative spectrum emitted by the plasma. According to the invention, the device includes a means (16) for repeatedly turning the plasma source (13) on and off. The device may further include an optical port (25) placed between the gas ionization system (8) and the freeze-drying chamber (1). The method of controlling the dehydration operation during a freeze-drying operation comprises an alternation of phases during which the plasma source (13) is turned on and phases during which the plasma source (13) is turned off.