[en] After dissolution of spent LWR fuel and conditioning of the dissolver solution for reprocessing, residues of varying composition are obtained. In the nuclear fuel reprocessing plant at Karlsruhe the residues contain 60-84% fines. In order to study their structure and composition samples from different dissolution campaigns were collected. One sample was prepared for characterisation by different surface analytical techniques and four samples were dissolved and analysed for their isotopic and elemental composition. Scanning electron microscopy and X-ray microanalysis showed two phases, a metallic phase mainly composed of Mo, Ru and Rh and an oxidic phase with the main components Zr and Mo, but containing also significant amounts of Pu. The analysis of the dissolved fines by isotope dilution mass spectrometry and inductively coupled plasma mass spectrometry revealed Mo to be the main component. Relatively high levels of Zr and Pu and low levels of Rh, Ru and Pd suggest that most of these fine residues had reprecipitated from the dissolver solution. (orig.)