A technique for the accurate measurement of the carbon kinetic isotope effects (KIEs) of the reactions of light non-methane hydrocarbons (NMHC) with OH radicals in the gas phase is presented. The method uses a variant of established relative rate techniques, which has been optimized for measurements of KIEs. The instrumentation consists of three basic components: (1) a gas-phase reaction chamber, (2) a gas chromatograph (GC) for separating the components of the reaction mixture, and (3) an isotope ratio mass spectrometer coupled on-line to the GC. Using this technique, measurements of hydroxyl radical reaction KIEs with C-2-C-9 saturated and unsaturated hydrocarbons having rate constants between 10(-11) and 10(-13) cm(3) molecule(-1)s(-1) are achievable. This method has been used successfully to measure the carbon KIE for the reactions of OH with a wide range of hydrocarbons in the gas phase.