Chemically activated CF2ClCF2CH3 and CF2ClCF2CD3, containing 98.5 and 100 kcal/mol of internal energy, respectively, were formed in the gas phase from the combination of CF2ClCF2 and CH3 or CD3 radicals, respectively. These radicals were generated from the UV photolysis of CF2ClCF2I and CH3I or CD3I. The decomposition products were CF2ClCF=CH2 (CF2ClCF=CD2) from a 2,3-HF (DF) elimination and CF3CF=CH2 (CF3CF=CD2) suggesting a I,3-HCl (DCl) elimination reaction. The 1,3-HCl elimination mechanism appears to be a two-step process; a 1,2-FCl rearrangement, producing CF3ClCFClCH3 (CF3CFClCD3), followed by a 2,3-HCl (DCl) elimination. Unimolecular rate constants for CF2ClCF2CH3 (CF2ClCF2CD3) were 5.3 +/- 2.1 x 10(5) s(-1) (1.8 +/- 0.7 x 10(5) s(-1)) for 2,3-HF (DF) loss and 3.6 +/- 1.4 x 10(4) s(-1) (2.3 +/- 0.9 x 10(4) s(-1)) for the 1,2-FCl rearrangement. The branching ratio was 13.5 +/- 3 (7.8 +/- 1.6) favoring the HF (DF) process. The isotope effect for 2,3-HF/DF was 2.9 +/- 0.6, while for the FCl rearrangement, it was considerably smaller at 1.5 +/- 0.3. The CF3CFClCH3 and the CF3CFClCD3, formed by the 1,2-FCl migration, react by loss of HCl (DCl) with rate constants of 2.1 +/- 1.3 x 10(7) s(-1) (7.9 +/- 4.8 x 10(6) s(-1)) and an isotope effect of 2.7 +/- 0.8. Theoretical rate constants, branching ratio and isotope effects were calculated using RRKM theory and density functional theory to compute all of the data necessary for the RRKM calculations. The agreement between the experimental and computed kinetic data suggests that the 1,3-HCl elimination is a two-step mechanism consisting of a 1,2-FCl rearrangement followed by a 2,3-HCl elimination.