The temperature dependence of the rate constants for the reactions F + CH3F and F + CH2F2 has been determined by a competitive method using the F + D-2 reaction as a reference reaction. Ratios of rate constants k(F+CH3F)/k(F+D2) and k(F+CH2F2)/k(F+D2) 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+D2) to obtain absolute values for k(F+CH3F) and k(F+CH2F2). The present results are compared with previous published results, which are available almost exclusively only for room temperature. (C) 2003 Elsevier Science B.V. All rights reserved.