In this study, we performed a partitioning of evapotranspiration (ET) under fully controlled conditions (climatic chamber)along growth of a tall fescue cover (Festuca arundinacea) into soil evaporation (Ev) and plant transpiration (Tr) by measuring their stable oxygen isotopic compositions (υET, υEv and υTr). We showed that it was possible, under the chamber's particular conditions, to realize the partition without (1) making the hypothesis of steady state transpiration usually done in the field, nor (2) calculating υEv as a function of air relative humidity, soil water and atmospheric vapour isotopic compositions. The contribution of Ev to total ET decreased over the experiment from 100% (bare soil) to 94% [16 days after the seeding (DAS), 83% (28 DAS), 70% (36 DAS) and 5% (43 DAS)]. Soil isotopic profiles calculated using a typical exponential-type expression and measured Ev flux were compared with the bare soil steady state measured profiles. Agreement between modelled and measured values was sensitive to soil tortuosity and kinetic fractionation values. Another significant result was highly enriched isotopic values estimated for soil water at the evaporation front [the surface under our experimental conditions (υsurf)]. The theoretical estimate of υsurf was about 16 enriched as compared to the values measured in the top 1 cm of the soil, raising important implications for the determination of υEv as a function of υsurf under field conditions. Our work also points out uncertainties related to the determination of partition values and isotopic composition measured in the field, a point that is often ignored in many papers on isotopic biogeochemistry applied to geochemical fluxes, although it can be important.