The ignition delay times of diluted reference gas/O-2/NO2/Ar mixtures (Phi = 0.25, 0.5, and 1.0, dilution 1:2 and 1:5, [NO2] = 20-250 ppm) were determined in a high-pressure shock tube. The temperature range was 1000 K <= T <= 1700 K at pressures of about 16 bar. The addition of NO2 leads to a significant reduction of the ignition delay times. This reduction increases with decreasing equivalence ratio. The effect of NO2 is well predicted by the NOx chemistry of different published reaction mechanisms. The differences in the predictions of the ignition delay times using a common hydrocarbon reaction mechanism and NOx subsystems of four published reaction mechanisms are negligible.