The kinetics of the reactions of ground state oxygen atoms with 1-pentene, 1-hexene, cis-2-pentene, and trans-2-pentene was investigated in the temperature range 200 to 370 K. In this range the temperature dependences of the rate constants can be represented by k ϭ AЈ T n exp(Ϫ E a Ј/RT) with AЈ ϭ (1.0 Ϯ 0.6) и 10 Ϫ14 cm 3 s Ϫ1 , n ϭ 1.13 Ϯ 0.02, E a Ј ϭ 0.54 Ϯ 0.05 kJ mol Ϫ1 for 1-pentene; AЈ ϭ (1.3 Ϯ 1.2) и 10 Ϫ14 cm 3 s Ϫ1 , n ϭ 1.04 Ϯ 0.08, E a Ј ϭ 0.2 Ϯ 0.4 kJ mol Ϫ1 for 1-hexene; AЈ ϭ (0.6 Ϯ 0.6) и 10 Ϫ14 cm 3 s Ϫ1 , n ϭ 1.12 Ϯ 0.05, E a Ј ϭ Ϫ 3.8 Ϯ 0.8 kJ mol Ϫ1 for cis-2-pentene; and AЈ ϭ (0.6 Ϯ 0.8) и 10 Ϫ14 cm 3 s Ϫ1 , n ϭ 1.14 Ϯ 0.06, E a Ј ϭ Ϫ 4.3 Ϯ 0.5 kJ mol Ϫ1 for trans-2-pentene. The atoms were generated by the H 2 -laser photolysis of NO and detected by time resolved chemiluminescence in the presence of NO. The concentrations of the O( 3 P) atoms were kept so low that secondary reactions with products are unimportant.