Reversed air staging, a method for reduction of N2O emissions, was studied in a 12 MW circulating fluidized bed boiler. The effect of combustor air-ratio, bed temperature, load and limestone addition ratio was investigated. The results indicate that if only the air distribution is changed, the emission of N2O can be decreased to one fourth compared to normal air staging at a temperature of 850 degrees C, with maintained low emissions of NO, SO2 and CO. With increased bed temperature, 870 degrees C, in combination with increased limestone addition, from Ca/S molar ratio = 3 to 4, it was possible to reduce N2O by 90 vol% with low emissions of NO, SO2 and CO. The method was less efficient at low load, because of the lower cyclone temperature. Also the effect of redistribution of some of the air added in the cyclone outlet to the cyclone inlet was studied. This did not, however, give any improvement in the boiler used compared to adding all of the air in the cyclone outlet. There was no significant difference in combustion efficiency between reversed air staging and normal air staging.