An engineering model that combines a simple mixing model and a detailed reaction mechanism has been used to investigate selective noncatalytic reduction (SNCR) of NO. In this process a jet of NH3 is injected at high temperatures into a flue gas containing NO and O-2. The mixing model used is based on the "maximum mixedness" model proposed by Zwietering. The chemical kinetic model of Miller and Glarborg was validated against experimental data obtained in a flow reactor over a range of NH3/NO/O-2 compositions corresponding to conditions ranging from early jet entrainment to full mixing between reactants. The effect of mixing was investigated on three different experimental scales: A laboratory-scale diffusion-mixing reactor, a bench-scale setup, and a full-scale grate-fired furnace. The results show that finite rate mixing affects the SNCR process efficiency at high temperatures where it may cause a narrowing or a widening of the temperature window, depending on the NO concentration. The effect of mixing on the selectivity for NO reduction could be modeled qualitatively in all scales with the proposed model, using mixing times estimated from simple jet correlations. Calculations for a full-scale wood-fired grate fumace indicate that for this system with NO concentrations of 50-100 ppm the initial segregation of reactants may enhance the process efficiency. In systems with higher NO levels finite rate mixing may have an adverse effect on the SNCR process.