Relative rate coefficients for the reactions of chlorine atoms with propene and 1-butene have been measured as a function of temperature (285-313 K) at 1 bar total pressure of air using FTIR as detection technique. Such studies are necessary to model the atmospheric chemistry of these alkenes and their impact on the air quality in areas where Cl chemistry is potentially important such as maritime and heavily polluted environments. The temperature dependence can be adequately described by the simple Arrhenius relationship and the following expressions in units of cm(3) molecule(-1) s(-1) have been obtained: k(propene) = (1.43 +/- 0.35) x 10(-14) exp[(2886 +/- 73)/T] and k(1-butene) = (1.60 +/- 0.81) x 10(-13) exp[(2235 +/- 152)/T] (errors are 2 sigma). (C) 2009 Elsevier B.V. All rights reserved.