The kinetics of the reactions of Br atoms with 2-methyl-2-butene (C5H10), 2,3-dimethyl-2-butene (C6H12), and 1-hexene have been studied by the mass spectrometry discharge-flow method at 0.5-2.0 Torr of He over the temperature range T = 233-320 K. The occurrence of both hydrogen atom abstraction and addition channels has been observed for all the reactions. The Arrhenius expressions k(1a) = (5.2 +/- 2.5) x 10(-16) exp{(1910 +/- 250)/T} and k(2a) = (8.4 +/- 6.3) x 10(-16) exp{(1950 +/-290)/T} cm(3) molecule(-1) s(-1) have been obtained (at 1 Torr total pressure of He) for the addition reactions Br + C5H10 <-> C5H10Br (1a,-1a) and Br + C6H12 <-> C6H12Br (2a,-2a), respectively. Considering k(7) and k(8) as the rate constants for the reactions of Br-2 with C5H10Br and C6H12Br, respectively, the following expressions have been determined: k(1a)k(7)/ k (-1a), = (1.4 +/- 0.8) x 10(-35) exp{(5250 +/-300)/T} and k(2),k(8)/k(-2a), = (6.2 +/- 6.8) x 10(-35) exp{(5190 +/- 510)/T} cm(6) molecule(-2) s(-1). Kinetic data have been also obtained for the H atom abstraction channels: k(1b) = (2.1 +/- 0.2) x 10(-11) exp{-(655 +/- 40)/T}, k(2b) = (2.4 +/- 0.6) x 10(-11) exp{-(75 +/- 130)/T}, and k(3b) = (3.25 +/- 0.4) x 10(-12) exp(-(425 +/- 70)/T) cm(3) molecule(-1) s(-1), for the reactions of Br with 2-methyl-2-butene, 2,3-dimethyl-2-butene, and 1-hexene, respectively. From this kinetic study, the enthalpies of reactions 1a and 2a and the heats of formation of the adduct radicals C5H10Br and C6H12Br have been estimated: Delta H degrees(1a) = -(10.0 +/- 1.6), Delta H degrees(2a) = -(9.8 +/- 2.1), Delta H degrees(f)(C5H10Br) = 6.8 +/- 1.6, and Delta H degrees(f)(C6H12Br) = 0 1 +/- 2 5 (in kcal mol(-1)).