This paper presents a computational study of ignition in nonpremixed counterflowing CO/H2versus heated air. To understand the chemical and dynamical mechanisms governing CO/H2ignition, we have examined the following features: the effect of H2concentration on the system's ignition characteristics, the dominant chemical kinetics just prior to ignition, the role of convection and diffusion with respect to chemical kinetics, and the dependence of ignition temperature on pressure variation. Three ignition regimes were identified, based upon the influence of H2concentration, which we have designated: 1) H2-dominating, 2) transition and 3) H2-catalyzing. In the H2dominated regime, relevant for high H2concentrations, the controlling ignition kinetics involve that of H2chemistry with minimal CO influence. In the transition regime, the participation of CO kinetics, especially through its exothermicity, allows the CO/H2system to be ignitable even when the analogous N2/H2system ceases to be ignitable. Sensitivity analysis shows that it is the competition between H and HO2chemistry in the two systems that determines whether or not the system is ignitable. Finally, in the H2-catalyzed regime, relevant for low concentrations, the dominant kinetics are found to include both H2and CO chemistry. Furthermore, by studying the effects of pressure on the ignition temperature in the of H2-catalyzed regime, three iginition limits corresponding to those of H2explosion limits are identifed, as is reasonable to expect.