The NO and N2O formation behavior of six biofuels (spruce wood, beech wood, alder wood, straw, malt waste, peat) was studied in a formation-rate unit under conditions relevant to a fluidized-bed combustor and a grate-furnace. The concentrations of CO2, CO, CH4, other hydrocarbons, NO, N2O, HCN, and NH3 were measured in the flue gas, shortly after the burning fuel particles. Most of the fuel nitrogen was released during devolatilization (66-75%). Relatively high conversions to NO were found. N2O was formed but also rapidly destroyed by the reaction: N2O + H --> N-2 + OH. HCN was also formed in quantities similar to NH3 even during wood combustion. The HCN/NH3 ratios seemed to depend on the fuel H/N ratios. The experimental results supported the hypothesis that the nitrogen of wood and other biofuels also exists in heterocyclic structures. With the proposed NO and N2O formation mechanism the effects of fuel nitrogen content, temperature and oxygen partial pressure can be explained.