Background and aims In forestry-drained peatlands, drying leads to changes in C cycling which could affect peat δ13C. Furthermore, the δ13C profile of the entire peat column may reveal effects of earlier climatic periods. Methods We measured peat δ13C and C inventories in adjacent peat profiles, two collected from undrained and two from the drained side of a bog that was partially ditch-drained 37 years earlier. The cores were sliced into 10-cm subsamples for analyses; matching of the profiles based on surface levelling, peat stratigraphic correlation and a horizontal ash layer found in both profiles. Results Surface subsidence of 30 cm was observed in the dried site and the uppermost 160 cm in the undrained site contained an excess of 5.9 kg m−2 of C compared with the corresponding strata of the ditch-drained site. The δ13C values increased but markedly only in the thin surface layer of the drained site, indicating low δ13C of the missing C (ca. –30‰). In the deeper strata, dating to Mid-Holocene, high dry bulk density, C%, N%, humification index and low C/N ratio were connected to low δ13C of peat. Conclusions Drainage of 37 years increased δ13C values in the upper peat profile of the drained bog and led to the selective loss of 13C depleted C. Results indicate that C balance studies can be aided by C isotope analyses. Low δ13C values in the peat profile indicate the existence of a wet fen stage during the moist and warm period during Mid-Holocene.