In order to relate the isotopic composition of soil organic matter to parent vegetation and soil type, we measured the carbon stable isotope ratios of tree leaves and soil C and 14 locations in a temperate forest. Sites were selected over a wide variety of soil types and related vegetation associations, but within a single regional climate. The δ13C of the bulk leaf material falling on the soil varied among sites, ranging from —29.5 to —26%. Within this range, differences up to 1.5% were attributable to differences in three species. Further differences were related to site. The δ13C values of soil organic matter cover the range —29.8 to 24.3% and varied with both parent vegetation δ13C and depth. Transformation and decay of organic C within the litter layers led to no isotope enrichment, irrespective of vegetation and humus type: mull, hydromull, moder, or anmoor. The difference between woody and leaf material in litter was low, 0.23 ± 0.35% (mean ± SD). On the other hand δ13C in mineral horizons always increased with depth, reaching values richer than litter by 1.0 ± 0.5% at 20 cm and up to +1.5% at 1 m. The trend was independent of the soil physico—chemical characteristics, whether aerobic, anaerobic, eutrophic, oligotrophic, or podzolic. Isotope ratios of soil organic C in forests appear to provide useful estimators of tree isotopic composition. When compared to the measurements of the vegetation, they offer the advantage of an easily obtained value that naturally integrates over plant material, time and, eventually, local area.