The correlation between the catalytic activity towards N2O decomposition and fuel type was studied for the bed materials sampled from the bottom bed of two industrial CFB boilers, a 12 MWth and a 550 MWth, burning biomass fuels and wastes, alone or as a mixture. It was found that the elemental composition of the surface of the bed material particles changed according to the composition of the ash from the parent fuel. The measured catalytic activity of the bed material samples increased with the amount of the catalytically active oxides (CaO, MgO, Fe2O3, Al2O3). In the case of limestone addition, the activity of the bed material was influenced by both the elemental composition of the fuel, and the ratio between lime and sulfated lime. (c) 2004 Elsevier B.V. All rights reserved.