The thermal decompositions of 1,1,1-trifluoroethane (CH3CF3) and pentafluoroethane (CHF2CF3) were studied by using a turbulent flow reactor at atmospheric pressure over the temperature ranges of 1213-1333 K and 1273-1373 K. The rate coefficients for these thermal decompositions were determined from the first-order decays of the reactants to be k(CH3CF3) = 10(12.9 +/- 1.2) exp[-d276 +/- 29) kJ center dot mol(-1)/RT] and k(CHF2CF3) = 10(13. 9 +/- 1.4) exp [-(324 +/- 36) kJ center dot mol(-1)/RT]s(-1). To examine the reaction paths, we identified the decomposition products using gas chromatography-mass spectrometry (GC-MS) and performed ab initio MO calculations. These results showed that the sequential HF elimination reactions, CH3CF -> CH2CF2 + HF and CH2CF2 -> CHCF + HF, were favorable for the CH3CF3 pyrolysis. On the other hand, four initial steps: CHF2CF3 -> CF3CF + HF, CHF2CF3 -> CF2CF2+ HF, CHF2CF3 -> CHF3 + CF2 and CHF2CF3 -> CHF2 + CF3, were possible for the CHF2CF3 pyrolysis. Such a difference in the reaction paths between the CH3CF3, and CHF2CF3 pyrolyses can be explained by fluorine hyperconjugation and by repulsion between the fluorine atoms on the 1- and 2-carbons of each hydrofluoroethane.