The use of stable isotopes to estimate evapotranspiration "ET# ~uxes from vegetated areas is increasing[ By complementing conventional net ~ux measurements "gradient or eddy correlation techniques#\ isotope analyses can allow partitioning ET between its gross components\ soil evaporation and leaf transpiration[ Isotopic analyses of atmospheric water vapour above canopies can also constrain\ or provide alternatives for estimating ET[ A brief discussion of the isotope approach is aimed at highlighting some of the uncertainties that require further research[ We also demonstrate _rst\ the application of combined concentration and isotopic gradient analysis of atmospheric water vapour above crop _elds in order to estimate ET ~uxes and its gross components "soil evaporation was estimated at 0=4Γ2=4) of mid!day ET ~ux in a mature wheat _eld#[ Second\ we demonstrate the potential in monitoring d ss -d L \ the di}erence between predicted and measured leaf water d 07 O values\ as an indicator of seasonal variations in canopy!scale transpiration in a desert ecosystem "linear correlation between this indicator and conventional ET measurements was observed#[ Improving our analytical capabilities for high!precision isotopic analysis of very small water vapour samples was a limiting factor in the above applications and a method for pyrolysis and on!line 07 O analysis of 9=1Γ1 mL water samples is described[