This paper reports the kinetic expressions for the catalytic activities toward N2O decomposition on three particle size fractions of the bottom bed materials, sampled from two industrial circulating fluidized-bed (CFB) boilers, a 12 MWth and a 550 MWth, burning biomass fuels and wastes. In addition, the kinetic expressions are derived for the activity of bed materials in the presence of 15 vol % of water vapor. The catalytic activity of the bed materials was determined by measuring the conversion of N2O in a laboratory fixed bed quartz reactor in the temperature range from 600 to 910 degrees C. The composition and morphology of the bed material were characterized using N-2-physisorption, An X-ray fluorescence (XRF), and a scanning electron microscope coupled with an energy-dispersive X-ray analyzer (SEM/EDXA). For bed-material samples from the 12 MWth CFB boiler, it was found that the activity of different size fractions decreased when the particle size increased from 125-297 to 355-500 mu m. The effect was more pronounced for the sample with highly reactive ash originating from municipal sludge. For bed-material samples from the 550 MWth CFB boiler, activities of different size fractions were mostly dependent on calcined limestone concentration. The presence of water vapor (0-30 vol %) was shown to inhibit the activity of bed materials considerably. Decrease in the activity of bed materials was significantly related to an increase of water to 15 vol %, but from 15 to 30 vol % there was only a minor further decrease.