This study was undertaken to develop a better understanding of how intake charge dilution by various gases affected nitric oxide (NO) emission from a single-cylinder spark ignition engine. Carbon dioxide, nitrogen, helium, argon, steam, and exhaust gas were individually added to the intake charge of a propane-fueled, single-cylinder engine operated at constant speed and load. Nitric oxide emission was reduced in all cases. The gases with higher specific heats gave larger NO reductions. The product of diluent flow rate and specific heat correlated with NO reduction. The effects of diluents on calculated combustion temperature, mbt spark timing, and fuel consumption are also presented and discussed.