Analytical estimates of NOx formation as a function of peak flame temperature and residence are potentially useful in the conceptual design studies of combustors for low NOx emissions characteristics. It is apparent that the accuracy of these design studies depends upon the actual flow conditions and the variations of the time and temperature occurring within the combustor relative to the estimates.An experimental program was undertaken to determine the limitations, as imposed by practical combustor and fuel systems designs, on the potential reduction of NOx emissions based on flame temperature and residence time predictions. A premix combustor designed to operate at an equivalence ratio of 0.75 was tested with an ideally mixed gaseous fuel. This produced data demonstrating the lower limits of NOx that might be attained and is compared to analytical model predictions. Further, burner designs incorporating lean equivalence ratios, premixing, and rapid mixing concepts were also tested. The results of the program indicate that the effects of non-uniform partially vaporized fuel-air mixtures as well as restrictive mixing rates attainable in practical combustors severely limit the full attainment of theoretical NOx reductions.