This study investigated the influence of climate on the carbon isotopic composition (δ13C) and oxygen isotopic enrichment (Δ18O) above the source water of different organic matter pools in European beech. In July and September 2002, δ13C and Δ18O were determined in phloem carbohydrates and in bulk foliage of adult beech trees along a transect from central Germany to southern France, where beech reaches its southernmost distributional limit. The data were related to meteorological and physiological parameters. The climate along the transect stretches from temperate [subcontinental (SC)] to submediterranean (SM). Both δ13Cleaf and Δ18Oleaf were representative of site-specific long-term environmental conditions. δ13C of leaves collected in September was indicative of stomatal conductance, vapour pressure deficit (VPD) and radiation availability of the current growing season. Δ18O was mainly correlated to mean growing season relative humidity (RH) and VPD. In contrast to the leaves, δ13Cphloem varied considerably between July and September and was well correlated with canopy stomatal conductance (Gs) in a 2 d integral prior to phloem sampling. The relationship between δ13C and Δ18O in both leaves and phloem sap points, however, to a combined influence of stomatal conductance and photosynthetic capacity on the variation of δ13C along the transect. Δ18Ophloem could be described by applying a model that included 18O fractionation associated with water exchange between the leaf and the atmosphere and with the production of organic matter. Hence, isotope signatures can be used as effective tools to assess the water balance of beech, and thus, help predict the effects of climatic change on one of the ecologically and economically most important tree species in Central Europe.