This paper, reports on a combined experimental and modeling investigation of NOx formation in laminar, ammonia-seeded, nitrogen-diluted, methane diffusion flames. The methane-ammonia mixture is a surrogate for biomass fuels, which contain significant fuel-bound nitrogen. The experiments use flue-gas sampling to measure the concentration of stable species in the exhaust gas. The computations use adaptive mesh refinement (AMR) to capture fine-scale features of the flame. The model includes a detailed chemical mechanism, differential diffusion, buoyancy, and radiative losses. The model shows good agreement with the measurements over the full range of experimental NH3 seeding amounts. As more NH3 is added, a greater percentage is converted to N-2 rather than to NO. The simulation results are analyzed to trace the changes in NO formation mechanisms with increasing amounts of ammonia in the fuel.