Radiocarbon analysis Of Soil CO2 can provide information on the age, source and turnover rate of soil organic C. We developed a new method for passively trapping respired CO2 on molecular sieve, allowing it to be returned to the laboratory and recovered for C isotope analysis. We tested the method on a soil at a grassland site, and using a synthetic soil created to provide a contrasting isotopic signature. As with other passive sampling techniques, a small amount of fractionation of the C-13 isotope occurs during sampling, which we have quantified, otherwise the results show that the molecular sieve traps a sufficiently large and representative sample Of CO2 for C isotope analysis. Since C-14 results are routinely corrected for mass-dependent fractionation, our results show that passive sampling of soil respiration using molecular sieve offers a reliable method to collect soil-respired CO2 for C-14 analysis. (C) 2009 Elsevier Ltd. All rights reserved.