The effects of a number of flame retardants (CF3I, CF3Br, and CF3H) on the high-temperature reactions of methane with air in a plug flow reactor are studied by numerical simulations using the Sandia Chemkin Code.(1) The dependence of (a) the ignition delay and (b) time for substantially complete reaction as a function of temperature and additive concentrations are calculated. In agreement with experiments, the ignition delay can be increased or decreased by the addition of retardants. The reaction time is always increased by additives. The mechanism for these effects has been examined. It is concluded that the ignition delay is controlled by the initial retardant decomposition kinetics, which releases active species into the system. These species can either terminate or initiate chains. The reaction time is largely a function of the concentrations of the active radicals H, OH, and O that are formed during the combustion process. It is shown that their concentrations, particularly those of H atoms, are lowered in the presence of the retardants. We find that the chemical mechanism governing reaction time is very similar to that which controls the flame velocity and a correlation between decreases in flame velocity and H-atom concentration is demonstrated. The calculations suggest that relative reaction time and I-I-atom concentrations should be effective measures for the estimation of retardant effectiveness.