Theory suggests that the level of enrichment of (18)O above source water in plant organic material (Delta) may provide an integrative indicator of control of water loss. However, there are still gaps in our understanding of the processes affecting Delta. One such gap is the observed discrepancy between modeled enrichment of water at the sites of evaporation within the leaf and measured enrichment of the leaf water as a whole (Delta(L)). Farquhar and Lloyd (1993) suggested that this may be caused by a Péclet effect. It is also unclear whether organic material formed in the leaf reflects enrichment of water at the sites of evaporation within the leaf or Delta(L). To investigate this question castor bean (Ricinus communis L.) leaves, still attached to the plant, were sealed into a controlled-environment gas exchange chamber and subjected to a step change in leaf-to-air vapor pressure difference. Sucrose was collected from a cut on the petiole of the leaf in the chamber under equilibrium conditions and every hour for 6 h after the change in leaf-to-air vapor pressure difference. Oxygen isotope composition of sucrose in the phloem sap (Delta(suc)) reflected modeled Delta(L). A model is presented describing Delta(suc) at isotopic steady state, and accounts for 96% of variation in measured Delta(suc). The data strongly support the Péclet effect theory.