A new model for describing the fuel-N oxidation to NO and N-2 in biomass combustion is presented. The formulation is based on the assumption that in biomass combustion the fuel-N is released as ammonia. The model consists of two formal reactions describing the oxidation of volatilized fuel-N: NH3 + O-2 = NO + H2O + 1/2 H-2, and NH3 + NO = N-2 + H2O + 1/2 H-2. The rate expressions were extracted from perfectly stirred reactor simulations using a comprehensive mechanism. The rate of NH3 oxidation to NO was determined by adding the net rate of all reactions involving NH3. The rate was determined at conditions where the formation of NO was dominating. The rate of the reaction between NH3 and NO was obtained by adding the net reaction rate of all reactions involving N-2. The following rate expressions were obtained: r(1) = 1.21 x 10(8)T(2)e(-8000/T) [NH3][O-2](0.5)[H-2](0.5), and r(2) = 8.73 x 10(17)T(-1)e(-8000/T)[NH3][NO]. The rates are given in mole.cm(-3).s(-1), the temperature in K and the concentrations in mole.cm(-3). The model is developed for use in CFD modeling of full-scale combustion devises. It describes the fuel-N chemistry well in flame-like conditions. In flue gas it predicts faster conversion than expected by a comprehensive mechanism.