Combined d C and d 18O analyses of water-soluble leaf and twig phloem material were used to determine intrinsic water-use efficiency (iWUE) and variability of stomatal conductance at different crown positions in adult European beech (Fagus sylvatica) and Douglas-fir (Pseudotsuga menziesii) trees. Simultaneous gas exchange measurements allowed evaluation of the differences in calculating iWUE from leaf or phloem water-soluble compounds, and comparison with a semi-quantitative dual isotope model to infer variability of net photosynthesis (An) between the investigated crown positions. Estimates of iWUE from d Ophloem, oxygen isotope composition of phloem exudates; Dw, leaf-to-air difference in the partial pressure of water vapour; ea, vapor pressure in the atmosphere; ei, vapor pressure in leaf intercellular air spaces; gm, mesophyll conductance to CO2; gs, stomatal conductance to H2O; gsc, stomatal conductance to CO2; iWUE, intrinsic water-use efficiency; iWUEcomp, intrinsic water-use efficiency derived from D OLP; Rsample, isotope ratio of the sample; Rstandard, isotope ratio of the corresponding standard; rs, stomatal resistance to water vapour; WSOM, water-soluble organic matter.