Oxygen isotopes are valuable tools for studying the gas exchange between terrestrial ecosystems and the atmosphere. We determined the δ18O signatures of soil CO2 fluxes from soil chamber measurements over the diurnal cycle in September 2000, May 2001 and July 2001 in a Sitka spruce plantation in Scotland. Concurrent estimates of the δ18O composition of soil water were obtained from soil samples collected in the vicinity of the chambers. The observed δ18O signatures of net soil CO2 fluxes were diurnally variable and strongly depleted compared to those expected from a simple evasion of respired CO2 at isotopic equilibrium with soil water. We then simulated the δ18O signatures of soil CO2 fluxes using a model of soil gas exchange that includes atmospheric invasion of CO2 with concurrent isotopic equilibration with soil water and evasion of the equilibrated CO2. This brought the modeled δ18O signatures closer to the observations, but complete agreement was only achieved when acceleration of isotopic exchange between CO2 and soil water by carbonic anhydrase activity was included. We hypothesize that carbonic anhydrase is present in the litter or surface soil layers. This introduces a feedback that can result in diurnally variable δ18O signatures of net soil CO2 fluxes. Such effects can only be captured in models that have an explicit description of the canopy air space with a variable δ18O signature of CO2.