The carbon kinetic isotope effects (KIEs) of the room temperature reactions of benzene and several light alkyl benzenes with OH radicals were studied in a reaction chamber at ambient pressure using gas chromatography coupled with online combustion and isotope ratio mass spectrometry (GCC-IRMS). The KIEs are reported in per mil according to $varepsilon$ (?) = (KIE - 1) ¿ 1000, where KIE = k12/k13. The following average KIEs were obtained, (all in ?): benzene 7.53 ¿ 0.50; toluene 5.95 ¿ 0.28; ethylbenzene 4.34 ¿ 0.28; o-xylene 4.27 ¿ 0.05, p-xylene 4.83 ¿ 0.81; o-ethyltoluene 4.71 ¿ 0.12 and 1,2,4-trimethylbenzene 3.18 ¿ 0.09. Our KIE value for benzene + OH agrees with the only reported value known to us <Rudolph et al., 2000>. It is shown that measurements of the stable carbon isotope ratios of light aromatic compounds should be extremely useful to study atmospheric processing by the OH radical.