Abstract. The authors examine the isotopic composition of leaf water, at natural abundance levels, as influenced by transpiration rate. The isotopic composition of water of wheat leaves (Triticum aestivum L. var. Aroona) was followed while their transpiration rate adjusted to ‘steady-state’ environmental conditions. Leaf diffusive resistance was modified by short-term salt treatment and by plant culture in either nutrient solution, free-draining sand, or vermiculite. Resultant changes in 18O and 2H in leaf water are described and fitted to the model of Leaney et al. (1985). The treatments with lower transpiration rates were found to have a greater fraction of their leaf water equilibrated with water vapour in the atmosphere. Comparable results were obtained with both 18O and 2H, with some differences being interpreted in terms of turbulence in the vapour diffusion path. The fraction of the leaf water equilibrated with the atmosphere varied between leaves of different ages. However, this may have been due to their different positions in the canopy.