Experiments were performed in a shock-tube facility to examine experimentally the kinetic effect, if any, of excess amounts of CO 2 as part of natural-gas-based fuel−oxidizer mixtures. An important aspect of these experiments was to also observe the role excess amounts of CO 2 play in causing nonidealities, particularly shock bifurcation, in shock-tube experiments using real (nondilute) fuel−air mixtures. Mixtures were composed of methane fuel at an equivalence ratio of 0.5 to represent a typical natural gas in a modified "air" mixture designed to study the effect of large levels of CO 2 dilution. These oxidizer compositions maintained constant levels of O 2 while exchanging N 2 for CO 2 in stages to give oxidizer mixture concentrations ranging from (0.21O 2 + 0.79N 2 ) to (0.21O 2 + 0.79CO 2 ). Low-pressure and high-pressure (near 1 and 10 atm, respectively) experiments were conducted over an approximate temperature range of 1450 to 1900 K. Results showed that the observed effect of CO 2 relating to reflected-shock bifurcation was quite significant, giving stronger bifurcation as amounts of CO 2 increased, as determined by a sidewall pressure transducer. Despite the presence of significant reflected-shock bifurcation in the mixtures containing high levels of CO 2 , the resulting ignition delay times were commensurate with the results expected if one were to assume the test conditions were at the inferred temperature and pressure immediately behind the reflected shock wave. That is, the main ignition events occurred in the gas closest to the endwall, where the effects of the shock bifurcation were minimal for the ignition delay time range of the present study. When the ignition delay times for mixtures with and without CO 2 dilution were compared, the effect of the CO 2 was minimal and within the uncertainty of the data, particularly for the experiments near 1 atm. A small effect of CO 2 addition was seen for the higher pressure near 10 atm, with a general increase in ignition delay time for the largest levels of CO 2 dilution. Predictions from a modern chemical kinetics model also showed a minimal effect of CO 2 addition on the methane ignition delay times, in agreement with the shock-tube data.