The temperature dependence of the rate constants for the reactions F+CH 3 F and F+CH 2 F 2 has been determined by a competitive method using the F+D 2 reaction as a reference reaction. Ratios of rate constants k F + C H 3 F /k F + D 2 and k F + C H 2 F 2 /k F + D 2 were determined by a discharge-flow-mass spectrometric technique at four temperatures in the ranges 189-298 and 182-298 K for the two reactions, respectively. These ratios were combined with recommended values for k F + D 2 to obtain absolute values for k F + C H 3 F and k F + C H 2 F 2 . The present results are compared with previous published results, which are available almost exclusively only for room temperature.