Temporal variations of carbon isotope composition of soil CO2 efflux (FS and δ13 CFS) at different time scales should reflect both temporal variations of the climate conditions that affect canopy functioning and temporal changes in the relative contribution of autotrophic respiration to total FS. A tunable diode laser spectrophotometer (TDLS) was installed in the Hesse forest (northeast of France) early during the 2007 growing season to determine the seasonal and daily variability in δ13 CFS. This method, based on the measurement of the absorption of an infrared laser emission at specific wave lengths of the 13 CO2 and 12 CO2, allows the continuous monitoring of the two isotopologues. The concentrations of the two isotopologues in FS were continuously monitored from June to November 2007 using chamber method and Keeling plots drawn from nocturnal accumulation of CO2 below the canopy. These TDLS measurements and isotope ratio mass spectrometer based Keeling plots gave very similar values of δ13 CFS, showing the reliability of the TDLS system in this context. Results were analysed with regard to seasonal and daily changes in climatic and edaphic variables and compared with the δ13 CofCO2 respired by roots, litter and soil incubated under controlled conditions. Pronounced daily as well as seasonal variations in δ13 CFS were recorded (up to 1.5‰). The range of variation of δ13 CFS was of the same order of magnitude at both diurnal and seasonal scales. δ13 CFS observed in the field fluctuated between values of litter and of root respiration recorded during incubation, suggesting that temporal (and probably spatial) variations were associated with changes in the relative contribution of the two compartments during the day and during the season.