Experiments were carried: out to study the influence of NO in air on the ignition temperature of hydrogen and hydrocarbons in a nonpremixed counterflowing system. These experiments were performed from 0.5 to 6 atm, with the NO concentration varying from 100 ppm to 15,000 ppm. It is shown that addition of a small amount of NO in air significantly reduces the ignition temperature of all fuels. For hydrogen, under certain pressures, NO eventually becomes an inhibitor at higher levels of addition. Thus there appears to exist an optimal NO concentration at which the catalytic effect is the most pronounced, and this optimal concentration was found to also depend on the system pressure. Numerical simulation was performed in the hydrogen case to better understand the kinetics of the observed phenomenon. It was found that at low NO concentrations, the ignition temperature was determined by the interaction of the H-2-O-2-NO subsystem, whereas at high NO concentrations the ignition temperature was mostly affected by the NOx chemistry. For hydrocarbons, the minimum temperature was much less pronounced and in most cases nonexistent. Furthermore, the extent of temperature decrease depended on the nature of the fuel.