The influence of bed-region stoichiome!ric ratio and fuel nitrogen content on the formation of gaseous species formed during grate combustion of biomass fuels is reported based on gas measurements made within the fuel bed. Three fuels were studied: two mixtures of pelletized bark and wood chips and one of pelletized straw. Experiments were performed in a vertical. cylindrical, laboratory-scale grate-furnace with 0.245 m i.d. and I .8 m height. Primary air was supplied through a grate consisting of a steel plate with 340 holes of 3.7 mm diameter. Secondary air was supplied 0.66 m above the grate. Gas analysis was performed for 0:. CO:. CO. Hz and NO. Results were compared with values calculated using a computer program for thermochemical equilibrium conditions. The measured contents of 0:. CO:. CO and Hz show good agreement with calculated equilibrium conditions at all bed region stoichiometries. A higher formation of NO was found for the straw fuel (0.58% fuel nitrogen) than for the bark/wood chip fuels (~0.25% fuel nitrogen). This is not in accordance with the thermochemical equilibrium calculations indicating that the formation of nitric oxide does not attain thcrmochcmical equilibrium and that the nitrogen content of the fuel has an influence on the amount of NO that is formed. The fuel nitrogen conversion to NO ranged from 3 to 20% at reducing conditions and from 20 to 40% at bed region stoichiometrics bctwccn I .BO and 1.25.