This paper reports the kinetic expressions for the catalytic activities toward NO reduction by CO of three bed materials from a 12 MWth circulating fluidized bed (CFB) boiler sampled during combustion of wood pellets and mixtures of wood pellets and two types of municipal sludge. The catalytic activity of the bed materials was determined by measuring the conversion of NO in a laboratory fixed bed quartz reactor in the temperature range from 870 to 1180 K. It was found that the activity of bed material increases upon addition of sludge to wood pellets. In the case of the bed material from the combustion of wood pellets, the catalytic activity was not significantly effected by the change of sample particle size, location in the boiler, or presence of char. Contrary, for the bed materials from the co-combustion of wood pellets and sludge, the particle size of 125-297 mu m, which was enriched in ash particles originating from sludge, showed higher catalytic activities as compared to fractions 250-355 or 355-500 mu m. The transport zone sample of the bed material from the co-combustion of wood pellets and sludge showed by a factor of 2 higher catalytic activities toward NO reduction by CO as compared to the sample from the bottom bed. After burning off the originally present char, the catalytic activity of the bed material from the co-combustion of wood pellets and sludge increased. The measured catalytic activities are comparable with the reported catalytic activity of the bed material from the combustion of bituminous coal and lower as compared to the catalytic activity of the bed material from the combustion of brown coal or coal chars. However, in the presence of 15 vol % of water vapor, the catalytic activity of the bed materials investigated in this study could be regarded as negligible.