Knowledge of the compartment-specific hydrogen isotopic composition (2H/1H) of cellular water (H+) is important as it is needed for calculation of isotopic fractionation. It is generally assumed (although nevertheless untested) that leaf water of higher plants is isotopically homogeneous within a single photosynthetic cell. To test this assumption, we compared the isotopic compositions of phytol and labdane (two diterpenes both synthesized in the chloroplast from the same precursor, geranylgeranyl pyrophosphate) extracted from leaves of C3 higher plants grown under controlled conditions. Phytol is on average 50‰ more depleted in 2H relative to labdane. After taking into account the kinetic isotopic effects associated with the biosyntheses from their common precursor geranylgeranyl pyrophosphate, a massive enrichment of stromal 2H+ by 860-1920‰ is inferred. Trans-thylakoid membrane proton pumping, which enriches stromal 2H+ and depletes lumenal 2H+ is the most plausible mechanism for such stromal 2H+ enrichment. However, we cannot exclude with rigor the possibility that stromal water isotopic enrichment results also from exporting 2H-depleted metabolites.