Phosphorus compounds have been reported to be a cause of increased deactivation rates of selective catalytic reduction (SCR) DeNOx catalysts in coal fired power plants. The deactivating behaviour of phosphorus was previously verified during lab and bench scale investigations. The results have been in good compliance with other research activities on similar topics. In order to compare the results of lab and bench scale tests with the actual operating conditions, measurements at two full scale power plants were carried out. The plants co-fired either meat and bone meal or sewage sludge as secondary fuel, while both represent high phosphorus fuels with a considerable amount of volatile fractions. The results of fuel and fly ash analysis showed an increased content of phosphorus during the co-combustion conditions. The analysis of the fine particulate matter showed a high phosphorus concentration on particles smaller than 0.9 mu m. A considerable phosphorus concentration on particle fractions smaller than 0.05 mu m is a clear indication of the presence of gaseous phosphorus compounds. These are expected to have a high deactivation potential on the SCR catalysts. (c) 2005 Elsevier Ltd. All rights reserved.