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The effect of large exhaust gas re-circulation (EGR) quantities on NOx production in a natural-gas-fueled direct-injection heavy-duty diesel engine has been tested over a range of speed, load, and timing in controlled experiments with a single-cylinder engine. At the highest EGR ratio, as much as 50% of the cylinder- out NOx was NO2. NOx results correlated well with oxygen mole fraction in the unburned gas because of the direct dependence of flame temperature on this quantity. Within the range of measurements, speed and load had little or no effect on the relationship between oxygen mole fraction and NOx production. A multi-zone model for estimating combustion rate, flame temperature, wall heat transfer, and NOx production from engine operating conditions and the record of cylinder pressure development with crank angle, was used to interpret experimental measurements.