The N2O concentration was measured in a circulating fluidized bed boiler of commercial size. Kinetics for N2O reduction by char and catalytic reduction and decomposition over bed material from the combustor were determined in a laboratory fixed bed reactor. The destruction rate of N2O in the combustion chamber and the cyclone was calculated taking three mechanisms into account : reduction by char, catalytic decomposition over bed material, and thermal decomposition. The calculated destruction rate was in good agreement with the measured destruction of N2O injected at different levels in the boiler. The conclusion is that in the bottom part of the combustor, where the solids concentration is about 1000 kg/m(3) (voidage 0.6) and the char content in solids 2 wt %, heterogeneous reactions were the most important N2O destruction mechanisms. Reduction by char accounted for 80% of the N2O destruction, 20% was due to catalytic decomposition over bed material, and homogeneous thermal decomposition was negligible. However, at higher levels in the combustor, the solids concentration is lower : at the top 60% of the N2O destruction was due to thermal decomposition and in the cyclone heterogeneous destruction of N2O was insignificant. It was estimated that more than one-half of the formation of N2O in the combustion chamber takes place above the secondary air inlet.