The aim of the investigations described in this paper is to provide basic information on the interaction of phosphorus compounds with fly ash. The information shall give better knowledge on the deactivation of DeNO(x)-catalysts by phosphorus during co-combustion of sewage sludge or meat and bone meal. For the investigations, hard coal is doped with mineral or organic phosphates and sodium chloride. The fuel is burned in a plug flow reactor. Fly ash aerosols are collected at 900 degrees C and 420 degrees C. Enrichments of dopants on fine particulate matter indicate nucleation or condensation. This may significantly contribute to catalyst deactivation due to the high mobility and penetration capability of the small particles into the catalyst pores. The results show that the phosphorus concentration on the submicron particles is widely independent of the flue gas temperature. A strong relation of calcium and phosphorus is observed for the addition of both phosphates while sodium chloride shows an indirect influence only. Derived reactions of flue gas particles with sub-micron calcium phosphates indicate a contribution to catalyst deactivation at usual operating temperatures. (c) 2006 Elsevier Ltd. All rights reserved.