The effectiveness of various agents including N2, C2HF5, C3HF7, CF3Br, and CF3I was compared in suppressing the ignition of stoichiometric mixtures of methane/air, propane/air, and ethene/air. Measurements were made of the critical ignition temperature of the reactants as they flowed over a heated nickel surface. The results showed that CF3Br and CF3I were both highly effective suppressants of ignition for all fuels studied. This was not the case for N2, C2HF5, and C3HF7, which tended to have little effect on ignition unless very high agent concentrations were present, when ignition was not achieved. The effect of the average reactant velocity and the angle of approach of the reactants (relative to the heated metal surface) on the critical ignition temperature for stoichiometric ethene/air mixtures flowing over the heated nickel foil were also measured.