Flue gas recirculation (FGR) is a well-known method used to control oxides of nitrogen (NOx) in industrial burner applications. Recent small-and large-scale experiments have shown that introducing the recirculated Hot eases with the fuel results in a much greater reduction in NOx. per unit mass of gas recirculated, compared to introducing the flue gases with the combustion air. At present, however, there is no definitive understanding of why introducing the recirculated gases with the fuel is more effective than conventional FGR. The objective of the present investigation is to ascertain to what degree chemical kinetics and/or molecular transport effects can explain the differences in NOx reduction observed between fuel-side and air-side introduction of Rue gases by studying laminar diffusion flames. Numerical simulations of counterflow diffusion flames using full kinetics were performed and NOx emission indices calculated for various conditions, Studies were conducted in which N-2 diluent was added either on the fuel- or air-side of the flame for conditions of either fixed initial velocities or fixed fuel mass flux. Results from these simulation studies indicate that a major factor in diluent effectiveness is the differential effect on flame zone residence times associated with fuel-side versus air-side dilution. Experiments using laminar jet flames were conducted in which either the air or fuel stream was diluted with N-2. The experiments showed that fuel-side dilution results in somewhat greater NOx emission indices than ail-side dilution. The higher flame temperatures measured with fuel dilution appear to be the principal cause of the higher emissions. The results of both the numerical simulations and the experiments suggest that, although molecular transport and chemical kinetic phenomena are affected by the location of diluent addition depending on Row conditions, the dramatically greater effectiveness of fuel-side over air-side introduction of recirculated flue eases in practical applications likely results also from differences in turbulent mixing and heat transfer.