Lean-burn natural gas engines offer attractively low particulate matter emissions and enjoy higher efficiencies than their stoichiometric counterparts. However, even though oxides of nitrogen emissions can be reduced through operation at lambda ratios of greater than 1.3, catalysts cannot reduce the oxides of nitrogen emissions in the oxidizing exhaust environment. Exhaust Gas Recirculation (EGR) offers the potential to reduce engine out oxides of nitrogen emissions by reducing the flame temperature and oxygen partial pressure that encourages their formation during the combustion process. A comparative study involving a change in the nature of primary diluent (air replaced by EGR) in the intake of a Hercules, 3.7 liter, lean-burn natural gas engine has been undertaken in this research. The Hercules engine was equipped with a General Motors electronically controlled EGR valve for low EGR rates, and a slide valve, constructed in house, for high EGR rates. Both valves were calibrated to quantify the EGR based upon pressures before and after the valves. The engine was mapped to find out the best possible combination of speed and load where replacement of excess air by EGR could be achieved. The combination of 2600 rpm and 70 ft-lb was found to be the best to achieve replacement. Replacement from a stock air-fuel equivalence ratio (lambda) of 1.32 till a stoichiometric lambda of 1.0 yielded favorable results. These replacement runs were not optimized for ignition timing and in consequence timing excursions were conducted for the replacement runs at lambda of 1.1, 1.2 and 1.32 and resulting emissions reported.