Reduction of NO emissions during combustion of wood-chips in a 12 MW circulating fluidized bed boiler was investigated by lowering the air-ratio in the combustor through secondary air addition in the cyclone outlet. This mode of operation was studied at three loads and also in combination with secondary air addition in the cyclone inlet and/or at 2.2 m height. A significant reduction in NO emissions was obtained without high CO emissions. Important for the success of NO abating measures was that the CO emissions should not increase to high levels. Therefore the conditions for CO burnout were studied. The temperature in the duct downstream of the cyclone outlet, the "exit chamber", was found to be important, as the burnout of CO was not completed in the cyclone, not even under normal operating conditions. The explanation seems to be incomplete mixing of the gases entering the cyclone. There was a clear correlation between the temperature in the exit chamber and the CO emissions, and various measures taken to increase the temperature in the exit chamber were effective in reducing CO emissions.