A device for measuring the activity of a liquid in a complex medium and the associated method, the device comprising a sealed enclosure in which the complex medium is intended to be placed and means for maintaining this enclosure at a constant temperature T, characterized in that it comprises : a means (14) for modifying the total volume of the enclosure ; a pressure sensor (22) for measuring the total pressure in the enclosure ; and means (24) for calculating the activity of the liquid in the complex medium from the temperature T, from the total volume of the enclosure and from the total pressure in the enclosure.

device, complex medium, temperature