ABSTRACT. The stable-carbon kinetic isotope effects (KIEs) for the gas-phase reactions of isoprene, methacrolein (MACR), and methyl vinyl ketone (MVK) with OH radicals were studied in a 25 L reaction chamber at (298 ± 2) K and ambient pressure. The time dependence of both the stable-carbon isotope ratios and the concentrations was determined using a gas chromatography combustion isotope ratio mass spectrometry (GCC-IRMS) system. The volatile organic compounds (VOCs) used in the KIE experiments had natural-abundance isotopic composition thus KIE data obtained from these experiments can be directly applied to atmospheric studies of isoprene chemistry. All 13 C/ 12 C KIE values are reported as ε values, where ε = (KIE -1) × 1000‰, and KIE = k 12 /k 13 . The following average stable-carbon KIEs were obtained: (6.56±0.12)‰ (isoprene), (6.47±0.27)‰ (MACR), and (7.58±0.47)‰ (MVK). The measured KIEs all agree within uncertainty to an inverse molecular mass (MM) dependence of OH ε(‰) = (487±18)MM -1 ,which was derived from two previous studies [J.Geophys. Res. 2000, 105, 29329-29346; J. Phys. Chem. A 2004, 108, 11537-11544]. Upon adding the isoprene, MACR, and MVK OH ε values from this study, the inverse MM dependence changes only marginally to OH ε(‰) = (485±14)MM -1 . The addition of these isoprene OH ε values to a recently measured set of O3 ε values in an analogous study [Atmos. Environ. 2008, 42, 8728-8737] allows for estimates of the average change in the 12 C/ 13 C ratio due to processing in the troposphere.