In the City of Nuremberg, from 1972 to 1992 the sewage sludge used to be thermally stabilized in a modified Porteous process. In recent years the impact of this process on the concentration of various persistent contaminants in the sludge has been studied. In the process part the organic material in the sludge is decomposed and to the same extent an increase of the concentration of persistent components is found in the processed sewage sludge. The enrichment factors for heavy metals and PCB show a modest variation between 1.15 and 1.63 with a mean value around 1.3. In contrast to these data, for PCDD/F enrichment factors up to 8 are found with a strong variation between the congeners, and the highest enrichment factors to be found for tetra- and pentachlorinated dioxins. The thermal stabilization of sewage sludge seems to be a source of PCDD/F due to their secondary formation in the process.