DeNO(x) catalysts for the selective catalytic reaction (SCR) in coal fired power plants are deactivated by catalyst poisons such as As, Na, K, P, etc. The deactivation rate depends on the fuel quality and the concentration of catalyst poisons in the fuel. However, operational parameters such as 02 content, residence time, combustion and flue gas temperature also have an influence on decreasing catalyst activity. Equilibrium calculations were carried out to identify possible catalyst deactivation reactions. The calculations concentrated on the elements phosphorus and sodium as primary catalyst poisons in animal residues or sewage sludge. A set of deactivated catalyst samples from different power plants were analysed with respect to surface area, chemical composition and permeability. From the results obtained, deactivation mechanisms were derived which seem to cause increased catalyst deactivation due to P-rich secondary fuels. The samples showed a high P and alkali concentration on the surface. The surface area and the pore volume decreased. Compared to pure coal combustion this decrease was significant for meat and bone meal co-combustion and was also noticed for samples exposed to sewage sludge co-combustion. A correlation between relative activity and Na and P concentration is established. The results of the surface analyses indicate pore condensation as a major deactivation mechanism. (C) 2003 Elsevier B.V. All rights reserved.