The chemical structure of stoichiometric methane–oxygen–argon flames seeded or not with 1% heptafluoropropane (CF3CHFCF3) was measured by the molecular beam-mass spectrometry technique. Both flames were stabilized on a water-cooled flat-flame burner under low pressure (4.2 kPa). Mole fraction profiles were computed by a simulation code as well. Modeling of the unseeded flame was performed with an updated version of a mechanism issued from Warnatz. Two submechanisms were considered to model the chemistry of fluorinated species: (i) a mechanism proposed by Westbrook to model flame inhibition by CF3Br, (ii) a modified version of (i) validated recently by Sanogo in a modeling study of the effect of C2F6on a methane flame. Both submechanisms were compared on the basis of their reaction pathways. They have in common a key role played by CF2in the consumption of the fluorinated additive. The consumption of this radical forms CF that is consumed very slowly with Westbrook mechanism,in contradiction with experimental results. A better agreement is obtained with the modified version so that this study extends and confirms its validation.