Rate coefficients, k, for the gas-phase reactions of Cl atoms and NO3 radicals with 2,3,3,3-tetrafluoropropene, CF3CFdCH2 (HFO-1234yf), and 1,2,3,3,3-pentafluoropropene, (Z)-CF3CFdCHF (HFO-1225ye), are reported. Cl-atom rate coefficients were measured in the fall-off region as a function of temperature (220-380 K) and pressure (50-630 Torr; N2, O2, and synthetic air) using a relative rate method. The measured rate coefficients are well represented by the fall-off parameters k0(T) ) 6.5 × 10-28 (T/300)-6.9 cm6 molecule-2 s-1 and k∞(T) ) 7.7 × 10-11 (T/300)-0.65 cm3 molecule-1 s-1 for CF3CFdCH2 and k0(T) ) 3 × 10-27 (T/300)-6.5 cm6 molecule-2 s-1 and k∞(T) ) 4.15 × 10-11 (T/300)-0.5 cm3 molecule-1 s-1 for (Z)-CF3CdCHF with Fc ) 0.6. Reaction product yields were measured in the presence of O2 to be (98 ( 7)% for CF3C(O)F and (61 ( 4)% for HC(O)Cl in the CF3CFdCH2 reaction and (108 ( 8)% for CF3C(O)F and (112 ( 8)% for HC(O)F in the (Z)-CF3CFdCHF reaction, where the quoted uncertainties are 2σ (95% confidence level) and include estimated systematic errors. NO3 reaction rate coefficients were determined using absolute and relative rate methods. Absolute measurements yielded upper limits for both reactions between 233 and 353 K, while the relative rate measurements yielded k3(295 K) ) (2.6 ( 0.25) × 10-17 cm3 molecule-1 s-1 and k4(295 K) ) (4.2 ( 0.5) × 10-18 cm3 molecule-1 s-1 for CF3CFdCH2 and (Z)-CF3CFdCHF, respectively. The Cl-atom reaction with CF3CFdCH2 and (Z)-CF3CFdCHF leads to decreases in their atmospheric lifetimes and global warming potentials and formation of a chlorine-containing product, HC(O)Cl, for CF3CFdCH2. The NO3 reaction has been shown to have a negligible impact on the atmospheric lifetimes of CF3CFdCH2 and (Z)-CF3CFdCHF. The energetics for the reaction of Cl, NO3, and OH with CF3CFdCH2 and (Z)-CF3CFdCHF in the presence of O2 were investigated using density functional theory (DFT).