The NOx emissions of a UK-manufactured domestic/small-scale 50 kW(th) underfeed stoker biomass pellet boiler were experimentally investigated under different air staging configurations including varying primary to secondary air ratios and different heights of the secondary air inlets above the fuel bed. Two different commercial biomass pellet fuels, i.e. standard Class A wood pellets (fuel-N = 0.13 wt%) and clean waste wood pellets (fuel-N = 0.46 wt%), were used in these tests. The total excess air level of the boiler was kept constant for all air staging configurations. The experimental results show that even with the domestic/small-scale biomass boiler, air staging can be effective and lead to considerable NOx reductions, particularly with biomass fuels containing relatively high fuel-N. The height of the secondary air inlets above the bed was found to have important impacts on both NOx and CO emissions. However, the trade-off between NOx emissions and CO emissions needs to be carefully considered when air staging is used to control NOx emissions of domestic/small-scale biomass boilers. (C) 2012 Elsevier Ltd. All rights reserved.