This paper presents a computational study of ignition in nonpremixed counterflowing CO/H-2 versus heated air. To understand the chemical and dynamical mechanisms governing CO/H, ignition, we have examined the following features: the effect of H-2 concentration 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 H-2 concentration, which we have designated: 1) H-2-dominating, 2) transition and 3) H-2-catalyzing. In the H-2-dominated regime, relevant for high H-2 concentrations, the controlling ignition kinetics involve that of H-2 chemistry with minimal CO influence. Ln the transition regime, the participation of CO kinetics, especially through its exothermicity, allows the CO/H-2 system to be ignitable even when the analogous N-2/H-2 system ceases to be ignitable. Sensitivity analysis shows that it is the competition between H and HO2 chemistry in the two systems that determines whether or not the system is ignitable. Finally, in the H-2- catalyzed regime, relevant for low H-2 concentrations, the dominant kinetics are found to include both H, and CO chemistry. Furthermore, by studying the effects of pressure on the ignition temperature in the of H-2-catalyzed regime, three iginition limits corresponding to those of H-2 explosion limits are identifed, as is reasonable to expect.