The application of 13C/12C in ecosystem‐scale tracer models for CO2 in air requires accurate measurements of the mixing ratios and stable isotope ratios of CO2. To increase measurement reliability and data intercomparability, as well as to shorten analysis times, we have improved an existing field sampling setup with portable air sampling units and developed a laboratory setup for the analysis of the δ13C of CO2 in air by isotope ratio mass spectrometry (IRMS). The changes consist of (a) optimization of sample and standard gas flow paths, (b) additional software configuration, and (c) automation of liquid nitrogen refilling for the cryogenic trap. We achieved a precision better than 0.1‰ and an accuracy of 0.11 ± 0.04‰ for the measurement of δ13C of CO2 in air and unattended operation of measurement sequences up to 12 h. Copyright © 2008 John Wiley & Sons, Ltd.