While the reaction kinetics of atomic O in its ground state, (O3P), with hydrogenated ethylenes has been studied, only a limited number of relative rate constants have been reported for atomic O reactions with halogen substituted alkenes. There are only a few studies that have been performed with perhaloalkenes. We have investigated the reactions of O(3P) with CF3CH=CH2 and CF3CCl=CCl2 in a discharge flow tube apparatus at 298 K, in order to obtain the absolute rate coefficients for these reactions and to compare its values with previously reported results from relative rate measurements.