The carbon kinetic isotope effects (KIEs) of the room-temperature reactions of several light alkanes and ethene with OH radicals were measured in a 30 L PTFE reaction chamber at ambient pressure using gas chromatography coupled with online combustion and isotope ratio mass spectrometry (GCC-IRMS). For simplicity, KIEs are reported in per mil according to epsilon (parts per thousand) = (k(12)/k(13) - 1) x 1000. The following average KIEs were obtained, (all in parts per thousand): ethane, 8.57 +/- 1.95; propane,. 5.46 +/- 0.35; n-butane, 5.16 +/-0.67; methylpropane, 8.45 +/- 1.49; n-pentane, 2.85 +/- 0.79; methylbutane, 2.91 +/- 0.43; n-hexane, 2.20 +/- 0.07; n-heptane, 1.96 +/- 0.26; n-octane, 2.13 +/- 0.39; cyclopentane, 1.84 +/- 0.13; cyclohexane, 4.46 +/- 0.51; methylcyclopentane, 1.77 +/- 0.53; ethene, 18.6 +/- 2.9. As well, the room-temperature rate constant for the reaction of methylcyclopentane + OH, not previously reported in the literature, was determined using relative rates: (8.6 +/- 2.2) x 10(-12) cm(3) molecule(-1) s(-1), including the estimated 25% uncertainty in the rate constant for cyclopentane + OH. KIE values for propane, n-butane and n-hexane have been reported previously [J. Geophys. Res. [Atmos.] 2000, 105, 29329]. Our KIE for n-hexane is in agreement with the previous measurement, but our values for propane and n-butane are both higher. The dependence between the KIE and chemical structure is discussed, and a method for estimating unknown carbon KIEs for the reactions of light alkanes with OH radicals is presented. With only one exception, predictions using this method agree within a factor of 2 of the experimental KIE results.