Abstract: The invention relates to a device (7) for applying electromagnetic energy to a reactive medium, with said device being designed to be connected to an electromagnetic radiation generator using a means of transmission (4) of said radiation, with the device being remarkable in that it includes at least one loss transmission line (70) that features an interface that transfers electromagnetic energy towards the reactive medium, with said transfer interface being arranged so as to assure, throughout the length of the said line, a pre-determined distribution cross-section for the density of the electromagnetic energy delivered to said medium by said line. The invention also relates to a device for processing, by electromagnetic radiation, of a reactive medium including such an application device. The present invention can be applied to the sector of devices for thermal treatment of a reactive medium, specifically by microwave or high-frequency radiation.