The paper describes results of a field test program to investigate the usefulness of combustion modification in reducing NOx emissions from industrial boilers (ranging in size from 11 to 528 GJ/hr). The gaseous and particulate emissions from coal, oil, and natural-gas fuels were measured both before and after the combustion modification. Data were taken on particulate size as well as concentration. The principal combustion modification methods investigated included reduced excess combustion air, staged combustion air, recirculated flue gas, tuned burners, and reset burner registers. Staging was implemented by using overfire air ports or by turning off the fuel to some burners and increasing the fuel to others, thus creating zones of fuel-rich combustion. All of the combustion modification methods were effective to varying degrees in reducing the NOx emissions, and reductions of as much as 50% were obtained with several of the modifications. In most instances boiler efficiency was not degraded, although the particulate emissions increased by up to 50% in some cases. There was no substantive effect on the other pollutant emissions that were measured.