The kinetics of the reaction of chlorine atoms with isoprene (1) has been studied using the discharge-flow mass-spectrometric method : Cl + C5H8 (+M) --> C5H8Cl (+M) (la); Cl + C5H8 --> C5H7 + HCl (lb) As a result of direct and relative measurements, the overall rate constant k(1) = (6.7 +/- 2.0) x 10(-11) exp{(485 +/- 85)/T} cm(3) molecule(-1) s(-1) was obtained at a pressure of 1 Torr of helium over the temperature range 233-320 K. k(1) was found to be pressure-independent (within 10%) at T = 298 K in the pressure range 0.25-3 Torr. Both HCl and C5H8Cl adduct were detected as products of reaction 1. The measurements of the HCl yield as a function of temperature led to the branching fraction for the H-atom abstraction channel (Ib) : k(1b)/k(1) = (1.22 +/- 0.4) exp{-(595 +/- 90)/T} (P = 1 Torr, T = 233-320 K); k(1b)/k(1) = 0.169 +/- 0.022 at 298 K. The results are compared with those reported in a recent study(4) carried out at 298 K. Zn addition, the rate coefficient has been measured for the react:ion Cl + Br-2 --> BrCl + Br (2), which was used as the reference in the relative study of reaction 1. Combining the present results obtained in the range 233-320 K with those available from a previous study in the range 298-401 K-10, the following Arrhenius expression is recommended : k(2) = (2.3 +/- 0.4) x 10(-10) exp{(135 +/- 60)/T} cm(3) molecule(-1) s(-1).