Small-scale developments have been made to an off-the-shelf continuous-flow gas chromatography/isotope-ratio mass spectrometry (CF-GC/IRMS) system to allow high-precision isotopic analysis of methane (CH4) and carbon dioxide (CO2) at ambient concentrations. The repeatability (1σ) obtainable with this system is 0.05‰ for δ13C of CH4, 0.03‰ for δ13C of CO2, and 0.05‰ for δ18O of CO2 for ten consecutive analyses of a standard tank. An automated inlet system, which allows diurnal studies of CO2 and CH4 isotopes, is also described. The improved precision for CH4 analysis was achieved with the use of a palladium powder on quartz wool catalyst in the combustion furnace, which increased the efficiency of oxidation of CH4 to CO2. The automated inlet further improved the precision for both CH4 and CO2 analysis by keeping the routine constant. The method described provides a fast turn-around in samples, with accurate, reproducible results, and would allow a long-term continuous record of CH4 or CO2 isotopes at a site to be made, providing information about changing sources of the gases both seasonally and interannually. Copyright © 2005 John Wiley & Sons, Ltd.