Abstract. Three approaches for determining the stable isotopic composition (δ¹³C and δ¹⁸O) of soil CO efflux were compared. A new technique employed mini-towers, constructed of open-topped piping, that were placed on the soil surface to collect soil-emitted CO₂. Samples were collected along a vertical gradient and analyzed for CO₂ concentration and isotopic composition. These data were then used to produce Keeling plots to determine the δ¹⁸O and δ¹³C of CO₂ emitted from the soil. These results were then compared to the δ¹⁸O and δ¹³C of soil-respired CO₂ measured with two other techniques: (1) flux chambers and (2) estimation from the application of the diffusional fractionation factor to measured values of below ground soil CO₂ and to CO₂ in equilibrium with soil water δ¹⁸O. Mini-tower δ¹⁸O Keeling plots were linear and highly significant (0.81< r ² > 0.96), in contrast to chamber δ¹⁸O Keeling plots, which showed significant curvature, necessitating the use of a mass balance to calculate the δ¹⁸O of respired CO₂. In the chambers, the values determined for the δ¹⁸O of soil respired CO₂ approached the value of CO₂ in equilibrium with surficial soil water, and the results were significantly δ¹⁸O enriched relative to the mini-tower results and the δ¹⁸O of soil CO₂ efflux determined from soil CO₂. There were close agreements between the three methods for the determination of the δ¹³C of soil efflux CO₂. Results suggest that the mini-towers can be effectively used in the field for determining the δ¹⁸O and the δ¹³C of soil-respired CO₂.