Rate constants for the reactions of hydroxyl radicals with CH3CF3 (HFC-143a) and CHF2CH2CF3 (HFC-245fa) have been measured using the flash photolysis resonance fluorescence technique over the temperature range 273-370 K. A data analysis procedure is presented which should minimize rate constant errors introduced by the possible effects of radical diffusion. The following Arrhenius expressions have been derived for the reactions of OH with CH3CF3 and CHF2CH2CF3, respectively (in units of cm(3) molecule(-1) s(-1)) : (0.95(-0.17)(+0.21)) x 10(-12) exp{-1979 +/- 65)/T} and (0.632(-0.078)(+0.089)) x 10(-12) exp{-1331 +/- 43)/T}. With these values, the atmospheric lifetimes for the two HFCs have been estimated to be 51 and 7.4 years, respectively. An error analysis is presented from which the rate constant uncertainty at any temperature can be calculated.