The contribution of leaf litter decomposition to total soil CO2 efflux (FL/F) was evaluated in a beech (Fagus sylvatica L.) forest in eastern France. The Keeling-plot approach was applied to estimate the isotopic composition of respired soil CO2 from soil covered with either control (−30.32‰) or 13C-depleted leaf litter (−49.96‰). The δ13C of respired soil CO2 ranged from −25.50‰ to −22.60‰ and from −24.95‰ to −20.77‰, respectively, with depleted or control litter above the soil. The FL/F ratio was calculated by a single isotope linear mixing model based on mass conservation equations. It showed seasonal variations, increasing from 2.8% in early spring to about 11.4% in mid summer, and decreasing to 4.2% just after leaf fall. Between December 2001 and December 2002, cumulated F and FL reached 0.98 and 0.08 kgC m−2, respectively. On an annual basis, decomposition of fresh leaf litter accounted for 8% of soil respiration and 80% of total C loss from fresh leaf litter. The other fraction of carbon loss during leaf litter decomposition that is assumed to have entered the soil organic matter pool (i.e. 20%) represents only 0.02 kgC m−2.