The flux (R s ) and carbon isotopic composition (δ 13 C Rs ) of soil respired CO 2 was measured every 2 h over the course of three diel cycles in a Mediterranean oak woodland, together with measurements of the δ 13 C composition of leaf, root and soil organic matter (δ 13 C SOM ) and metabolites. Simulations of R s and δ 13 C Rs were also made using a numerical model parameterised with the SOM data and assuming short-term production rates were driven mainly by temperature. Average values of δ 13 C Rs over the study period were within the range of root metabolite and average δ 13 C SOM values, but enriched in 13 C relative to the bulk δ 13 C of leaf, litter, and roots and the upper soil organic layers. There was good agreement between model output and observed CO 2 fluxes and the underlying features of δ 13 C Rs . Observed diel variations of 0.5 ‰ in δ 13 C Rs were predicted by the model in response to temperature-related shifts in production rates along a ∼3 ‰ gradient observed in the profile of δ 13 C SOM . However, observed δ 13 C Rs varied by over 2 ‰, indicating that both dynamics in soil respiratory metabolism and physical processes can influence short-term variability of δ 13 C Rs .