In previous investigations of co-combustion process characteristics, only minor emphasis was put on the fine particulate matter evolution in power plants. Contrary to this, work at the IVD (Institute of Process Engineering and Power Plant Technology) was aimed at determining the influence of the supplementary fuel sewage sludge on fine particulate matter. Extensive combustion tests were performed for three different input shares of sewage sludge to the test facility. The energy input was kept constant at 300 kW and energy shares were varied from 0%th sewage sludge to 5%th and 15%th. The fine particulate matter content was investigated at four different sampling locations in the combustion chamber. Two additional sampling locations were located in a cooler section of the facility, one past the air preheater and a second one past the cyclone of the facility. The particle samples withdrawn from the facilities, six sampling locations, were sampled by a low-pressure impactor with a sampling range between approx. 0.03 and 10 Inn. Investigations were performed for similar energy inputs ( approximate to 300 kW), residence times (between approximate to 1_5 and approximate to 10 s for the different sampling locations), temperatures and 0, concentrations in the off-gas ( approximate to 3.5 vol.%) but differing amounts of ash fed to the combustion chamber by the fuels (0%, 56% and 82% of the overall ash). This paper gives a clear indication on the positive effect co-combustion has on the fine particulate matter fraction transported to the particle separation device of the facility. The increase in the sewage sludge share of the overall energy input not only resulted in overall higher ash loads but also caused a decrease of the concentration of the finest particulate matter concentration ( < 1 μm) entrained in the flue gas. In the combustion chamber, the condensation of a specie with medium volatility was seen in the temperature region of 870 to 930 &DEG;C. The boiling point of Zn, enriched by 10 times in the sewage sludge and known to condense in the fraction of approx. 30 nm from combustion processes, is at 907 &DEG;C. It is therefore expected to be a considerable fraction of the newly condensed particle. In the later stages of the process, the concentration of that particle size fraction decreases again, hinting a further growth of particulate matter. In the later stages of the process at temperatures of approximately 260 &DEG;C and below an additional compound with high volatility is found to be condensed and coagulated and is found in the fraction of approx. 80 nm. With the boiling point Of P2O5 in the temperature range ( &AP; 360 &DEG;C) prior to this point of particulate withdrawal and the result from the investigation and chemical analyses of the two samples (0%th and 15%th sewage sludge), a P compound could be verified. © 2004 Elsevier B.V All rights reserved.