The recent ban on the production of CF 3 Br has motivated a search for alternate agents for fire suppression: however, a replacement agent with all of the desirable properties of CF 3 Br is proving difficult to find. While most of the research has concentrated on other halogenated agents, there exist agents that are much more effective than CF 3 Br but about whose inhibition mechanism less is known. These agents typically involve a condensed phase. This paper investigates the behavior of iron pentacarbonyl(Fe(CO) 5 ), one of the most efficient inhibitors identified in the past, in methane/oxygen/nitrogen flames: measurments are reported for both diffusion and premixed flames. The reduction in the extinction strain rate and laminar burning velocity with addition of iron pentacarbonyl is determined over moderate ranges of fuel-air equivalence ratio and oxygen mole fraction X O2 . The flame and inhibitor locations in the counterflow diffusion flame experiments are varied to control the condensed- and gas-phase species transport rates to the flames and the chemical environment for their reaction.In the premixed flames, iron pentacarbonyl at low concentration reduces the burning velocity up to 100 times more effectively than CF 3 Br and shows a strong dependence on both and X o2 ; however, at Fe(CO) 5 mole fractions above a few hundred ppm, there is no additional reduction of the burning velocity. In diffusion flames, Fe(CO) 5 is again shown to be more effective at the lowest concentrations, but the inhibition effect does not level off at higher mole fractions as it does in the premixed flames. In addition, combinations of flame and inhibitor location in the counterflow flames have been found for which iron pentacarbonyl either provides negligible inhibition at any mole fraction or promotes the combustion. The possible mechanisms are discussed.