The stabilization and suppression behavior of a nonpremixed methane flame formed behind a backward-facing step in a combustion tunnel has been studied by impulsively injecting a gaseous fire-extinguishing agent (CF3Br) into the airflow. Optical observations, including a schlieren method, revealed that two distinct flame stabilization and suppression regimes, i.e., (I) rim-attached and (II) wake-stabilized, appeared as the mean air velocity was increased. The suppression in regime I occurred after immediate detachment of the flame at agent arrival, followed by blowout of the flame in the sheer layer downstream. The suppression in regime II took place as a result of consecutive extinction of the flames in the sheer layer and the recirculation zone, and the latter flame is controlling. The characteristic mixing (or residence) time in the recirculation zone, measured by the sodium D-line emission method, linearly depended on a simple similarity parameter, i.e., the step height divided by the effective mean air velocity. For long agent injection periods, the critical agent mole fraction at suppression approached a minimum value of similar to 0.025, which was identical to the value obtained using a counterflow diffusion flame. On the other hand, the critical agent mole fraction at suppression increased dramatically as the agent injection period was decreased below the characteristic mixing time. Under relatively high air velocities (regime II), the critical agent mole fraction at suppression normalized by the minimum value was a unique function of the agent injection period normalized by the characteristic mixing time and can be predicted using a theoretical expression. Furthermore, the total agent mass delivered can be minimized at the injection period near the characteristic mixing time. Published by Elsevier Science Inc.