The kinetics of the reactions O(3P) + CF2=CCl2 and O(3P) + CF3CF=CF2 were studied at room temperature in a discharge flow tube system. The overall rate constants based on the measured afterglow reactions were (3.10 ± 0.40) × 10-13 and (3.00 ± 0.60) × 10-14 cm3 molecule-1 s-1. respectively. The experiments were carried out under pseudo-first-order conditions with [O(3P)]0 K [alkene]0. These results are compared with previous relative measurements using different experimental techniques. The effect of substituent atoms or groups on the overall rate constants is analyzed in comparison with other alkenes in the literature.