The invention relates to a method and a device for determining the distribution of an X-ray fluorescence (XRF) marker (16) in a body volume (14). The body volume (14) is irradiated with a beam of rays (12) from an X-ray source (10) with a first ray component with a quantum energy just above and a second ray component with a quantum energy just below the K-edge of the XRF marker (16). Secondary radiation emitted from the body volume (14) is detected in a location-resolved way by a detector (30). To separate the X-ray fluorescence components in the secondary radiation from background radiation, the body volume is irradiated for a second time with a beam of rays from which the first ray component has been substantially removed by a filter (22) made from the material of the XRF marker.