A method and device for detecting and extracting a gaseous fluid contained in a closed loop operating on a Rankine cycle are provided. The loop includes a multiplicity of constituents with, successively, a circulation and compression pump for a working fluid, a heat exchanger associated with a hot source, an expansion machine, a cooling exchanger, a working fluid tank and circulation pipes connecting these constituents. The method and device measure the temperature (Tréelle) and the pressure (Préelle) of the working fluid at a point of the loop when this loop is at rest, and, as soon as the measured pressure (Préelle) exceeds a threshold value (Pliquide saturé) for a given ambient temperature (T), activate equipment for extracting the gaseous fluid in order to discharge it from the loop.