Using continuous-flow isotope ratio mass spectrometry, we have developed a new analytical system which enables us to determine the stable carbon isotopic composition of CH3Cl, CH3Br, and C2C5 saturated hydrocarbons in gas samples even if they contain substantial amounts of unsaturated hydrocarbons, using an I2O5 reagent for their selective subtraction. The analytical precision of the δ13C determinations is better than 0.5‰ for >300 pmolC injections and better than 5‰ for 20 pmolC injections. Using the system, δ13C values for CH3Cl and CH3Br were found in burning exhaust that contain a substantial quantity of unsaturated hydrocarbons. CH3Cl and CH3Br measured in exhaust from burning rice plants exhibit highly 13C-depleted values of −56.6 ± 1.3‰ and −48.6 ± 3.9‰, respectively, while saturated hydrocarbons exhibit δ13C values (−26.4 to −28.9‰) that are comparable with the total δ13C value of the parent material (rice plant; −28.0 ‰). Using the system, we can determine the δ13C values of methyl halides and hydrocarbons in many kinds of gas samples. Copyright © 2005 John Wiley & Sons, Ltd.