In deciduous trees, the δ13C values of leaves are known to diverge during growth from those of woody organs. The main purpose of this study is to determine whether the divergence in δ13C between leaves and current-year twigs of Fagus sylvatica (L.) is influenced by changes (i) in the relative contents of organic matter fractions and (ii) in the δ13C of respired CO2. The δ13C values of bulk matter, extractive-free matter, lignin, holocellulose, starch, soluble sugars, water-soluble fraction and respired CO2, as well as their relative contents in bulk matter were determined. The δ13C values of biochemical fractions and respired CO2 showed very similar temporal variations for both leaves and twigs. Variations in bulk matter δ13C during growth were, therefore, poorly explained by changes in biochemical composition or in respiratory fractionation and were attributed to the transition from 13C-enriched reserves (mainly starch) to 13C-depleted new photoassimilates. The divergence between leaves and twigs was related to higher values of soluble sugar δ13C in twigs. However, the difference between lignin and holocellulose δ13C varied during growth. This phenomenon was attributed to the delay between holocellulose and lignin deposition. These results may have implications for analysis of organic matter δ13C in trees and forest ecosystems.