Hexafluoroacetone was cophotolyzed with acetone and with acetone-dg, and the rate of the unimolecular dehydrofluorination of the 1,1,1-trifluoroethanes formed from combination of CFg and CHg or CDs radicals was determined from the difluoroethene/trifluoroethane ratio. This ratio was found to depend on the extent of conversion because of secondary reactions. Extrapolations to zero conversion corrected for this effect. The measured kinetic isotope effect was found to be 2.84 ± 0.07. Values of the isotope effect and rate constants predicted by applying RRKM theory to two different models of• the activated complex show that both models predict results similar to observations.