An experimental and numerical study is performed to elucidate the mechanisms by which bromotriflouoromethane (CF3Br) extinguishes laminar nonpremixed flames. Experiments are conducted on methane flames stabilized in the counterflowing configuration. The fuel and oxidizer streams are diluted with nitrogen and the inhibitor CF3Br is added to one of the reactant streams. The volumetric flow rate of the reactants and their composition at extinction are used to calculate the characteristic strain rate at extinction employing a previously developed theory. The critical conditions of extinction are measured with various amounts of the inhibitor added to either the fuel or the oxidizer stream. Numerical calculations are performed using detailed chemistry and applying boundary conditions similar to those in the experiments. The chemical-kinetic mechanism employed in these calculations includes the detailed description of fuel oxidation and inhibition by CF3Br. In general, the critical conditions of extinction calculated using this detailed chemical-kinetic mechanism are found to agree reasonably well with the measurements. The influence of preferential diffusion is examined. The analysis of the experimental and numerical results show that CF3Br extinguishes the flame by chemical mechanisms. Also, CF3Pr is shown to be more effective in extinguishing the methane flame when it is added to the oxidizer stream rather than to the fuel stream.