Carbon dioxide respired by soils comes from both autotrophic and heterotrophic respiration. 13C has proved useful in differentiating between these two sources, but requires the collection and analysis of CO2 efflux from the soil. We have developed a novel, open chamber system which allows for the accurate and precise quantification of the delta13C of soil-respired CO2. The chamber was tested using online analyses, by configuring a GasBench II and continuous flow isotope ratio mass spectrometer, to measure the delta13C of the chamber air every 120 s. CO2 of known delta13C value was passed through a column of sand and, using the chamber, the CO2 concentration stabilized rapidly, but 60 min was required before the delta13C value was stable and identical to the cylinder gas (-33.3 per thousand). Changing the chamber CO2 concentration between 200 and 900 micromol.mol(-1) did not affect the measured delta13C of the efflux. Measuring the delta13C of the CO2 efflux from soil cores in the laboratory gave a spread of +/-2 per thousand, attributed to heterogeneity in the soil organic matter and roots. Lateral air movement through dry sand led to a change in the delta13C of the surface efflux of up to 8 per thousand. The chamber was used to measure small transient changes (+/-2 per thousand) in the delta13C of soil-respired CO2 from a peaty podzol after gradual heating from 12 to 35 degrees C over 12 h. Finally, soil-respired CO2 was partitioned in a labelling study and the contribution of autotrophic and heterotrophic respiration to the total efflux determined. Potential applications for the chamber in the study of soil respiration are discussed.