A new system for soil respiration measurement [P. Rochette, L.B. Flanagan, E.G. Gregorich. Separating soil respiration into plant and soil components using analyses of the natural abundance of carbon-13. Soil Sci. Soc. Am. J., 63, 1207–1213 (1999).] was modified in order to collect soil-derived CO₂ for stable isotope analysis. The aim of this study was to assess the suitability of this modified soil respiration system to determine the isotopic composition (δ¹³C) of soil CO₂ efflux and to measure, at the same time, the soil CO₂ efflux rate, with the further advantage of collecting only one air sample. A comparison between different methods of air collection from the soil was carried out in a laboratory experiment. Our system, as well as the other dynamic chamber approach tested, appeared to sample the soil CO₂, which is enriched with respect to the soil CO₂ efflux, probably because of a mass dependent fractionation during diffusion and because of the atmospheric contribution in the upper soil layer. On the contrary, the static accumulation of CO₂ into the chamber headspace allows sampling of δ¹³C-CO₂ of soil CO₂ efflux.