A Zone Model has been developed for the prediction of NO reduction by "reburning" in turbulent diffusion flames. The computations of fluid dynamics are decoupled from those of detailed combustion chemistry including those of the nitrogen species. The calculations begin with the computations of the stream function, heat release, temperature, and major species concentration distributions using a commercial CFD code and simple, one or two step chemistry. The modeled space is then subdivided into volume elements (zones) bounded by streamlines and some axial coordinate. The sizes of these zones are much larger than those of a computational grid so that it is practicable to use detailed chemistry for the calculation of progress of reaction within such a zone. In the experimental investigation natural gas with and without additive NH3 was used as "reburn" fuel. It was injected axially into the NO bearing combustion products of a 335 kW natural gas-air flame to obtain a parabolic flow which can be modeled more easily. Detailed in-flame measurements permitted critical tests of the prediction method by the comparison of experimental and calculated data.