We investigated plant-water sources of an emblematic refugial population of Fagus sylvatica (L.) in the Ciron river gorges in South-Western France using stable isotopes. The stable isotopes of water are a powerful tracer of water fluxes in the soil-plant-atmosphere continuum. It is generally assumed that no isotopic fractionation occurs during root water uptake, and that xylem water isotopes effectively reflect source water isotopes. However, recent studies showed that under certain conditions the isotopes in plant water do not reflect any of the potential sources considered. Highly resolved datasets covering a range of environmental conditions could shed light on possible plant-soil fractionations processes. In this study, the hydrogen (δ 2 H) and oxygen (δ 18 O) isotope compositions of all potential tree water sources and xylem water were measured bi-weekly over an entire growing season. Using Bayesian isotope mixing models (MixSIAR), we then quantified the contribution of the considered sources to xylem water of F. sylvatica and Quercus robur (L.) trees. Based on δ 18 O data alone, both species used a mix of top and deep soil water over the season, with Q. robur using soil water relatively deeper than F. sylvatica . The contribution of stream water appeared to be marginal despite the proximity of the trees to the stream, as already reported for other riparian forests. Xylem water δ 18 O could always be interpreted as a mixture of deep and shallow soil waters, but the δ 2 H of xylem water was often more depleted than any other possible water source. We argue that an isotopic fractionation in the unsaturated zone and/or within the plant tissues could underlie this unexpected δ 2 H depletion of xylem water, as already observed in halophytic and xerophytic species. By means of a sensitivity analysis, we found that the estimation of plant-water sources using isotope mixing models was largely affected by this isotopic δ 2 H depletion. A better understanding of what causes this isotopic separation between xylem and source water is urgently needed.