The effect of oxygen mole fraction on NOx emission index was investigated for laminar, nonpremixed CH4-O-2/N-2 flames. For O-2 mole fractions between 21% and 100%, stoichiometric and overventilated flames displayed maximum NOx emission indices at O-2 contents near 60%. EINOx values were less than those at 21% O-2 only for O-2 contents greater than similar to 95%. Overventilation (4% excess oxygen) increased EINOx for all O-2 contents less than 95%.