The reactions of Br atoms with ethene and trans-2-butene have been studied by the mass spectrometry discharge-flow method, at a total pressure of helium 0.5-2.0 Torr in the temperature range 233-320 K. The expression k(1)degrees (3.0 +/- 1.2) x 10(-33) exp{(730 +/- 220)/T} cm(6) molecule(-2) s(-1) has been obtained for the low-pressure limit of the rate constant for the addition reaction Br + C2H4 + M <-> C2H4B4 + M (1,-1). The expressions k(1)/k(5)/k(-1) (3.1 +/- 0.6) x 10(-36) exp{(3540 +/- 280)/T} cm(6) molecule(-2) s(-1) and k(1)k(6a)/k(-1) = (1.8 +/- 0.9) x 10(-35) exp{(3390 +/- 270)/T} cm(6) molecule(-2) s(-1) have been obtained for the combinations of the rate constants of reactions (1,-1) and reactions Br-2 + C2H4Br --> Br + C2H4Br2 (5) and Br + C2H4Br --> C2H4Br2 (6a). For the reaction of Br atoms with trans-2-butene, both the addition channel, Br + C4H8 <-> C4H8Br (2a,-2a), and the H atom abstraction channel, Br + C4H8 --> C4H7 + HBr (2b) have been observed and the following kinetic data were obtained: k(2a)degrees = (2.1 +/- 0.8) x 10(-32) exp{(1300 +/- 90)/T} cm(6) molecule(-2) s(-1), k(2a)k(9)/k(-2a) = (3.8 +/- 1.7) x 10(-36) exp{(4630 +/- 230)/T} cm(6) molecule(-2) s(-1) (where k(9) is the rate constant of the reaction C4H8Br + Br-2 --> C4H8Br2 + Br) and k(2b) = (5.8 +/- 1.7) X 10(-12) exp{-(960 +/-160)/T} cm(3) molecule(-1) s(-1). Thermochemical data around room temperature could be derived from these kinetic measurements; the enthalpies of reactions 1 and 2a are Delta H(1)degrees = -(6.8 +/- 1.6) kcal mol(-1) and Delta H(2a)degrees = -(8.8 +/- 1.5) kcal mol(-1), and the heats of formation of the adducts C2H4Br and C4H8Br are Delta H(f)degrees(C2H4Br) = 32.4 +/- 1.6 kcal mol(-1) and Delta H(f)degrees(C4H8Br) = 20.5 +/- 1.8 kcal mol(-1).