The kinetic isotope effects (KIEs) for several ozone-alkene reactions in the gas phase were studied in a 30 L PTFE reaction chamber. The time dependence of the stable carbon isotope ratios and the concentrations were determined using a gas chromatography combustion isotope ratio mass spectrometry (GCC-IRMS) system. The following average KIE values were obtained: 18.9 ¿ 2.8 (ethene), 9.5 ¿ 2.5 (propene), 8.7 ¿ 1 (1-butene), 8.1 ¿ 0.4 (E-2-butene), 7.9 ¿ 0.4 (1,3-butadiene), 6.7 ¿ 0.9 (1-pentene), 7.3 ¿ 0.2 (Z-2-pentene), 6.7 ¿ 0.7 (cyclopentene), 6.1 ¿ 1 (isoprene), 5.0 ¿ 0.7 (1-hexene), 5.6 ¿ 0.5 (cyclohexene), and 4.3 ¿ 0.7 (1-heptene). These data are the first of their kind to be reported in the literature. The ozone-alkene KIE values show a systematic inverse dependence from alkene carbon number. Based on the observed KIEs, the contribution of ozone-alkene reactions to the isotopic fractionation of alkenes in the atmosphere can be estimated. On average this contribution is generally small compared to the impact of reaction with OH radicals. However, when OH-concentrations are very low, e.g. during nighttime and at high latitudes in winter, the contribution of the ozone reaction dominates and under these conditions the ozone-alkene reaction will have a clearly visible impact on the stable carbon isotope ratio of atmospheric alkenes.