A tentative model was developed to predict the flux of tritiated water vapor from air to plant leaves, in retation to leaf stomatal resistance and temperature. The validity of this model was investigated by field experiments performed in autumn and spring, in order to meet different meteorological and physiological conditions. In these experiments, grape leaves were exposed to a constant level of tritiated water vapor. Foliar samples were collected at different times during and after exposure and were analysed for their tritium content. The tritium concentrations found in leaf tissue-water are in good agreement with the theoretical estimations, which indicates that the model is probably adequate for the description of THO movement and distribution.