There are indications that the adsorption of water-soluble gases like NH3 and SO2 on a leaf surface may be compared with the dry deposition on a freely accessible water layer. The aim of our study was to quantify the thickness of the apparent water layer on the leaf surface. The thickness was calculated from NH3 adsorption data on leaves. Also we performed weighing experiments with dried leaves of different plant species at relative air humidities of 20 and 95% (at 20oC), respectively. From the increase in weight the water film thickness was calculated. The thickness calculated from NH3 adsorption data strongly depends on air humidity and varied from 10 m at low relative humidities to 100 m at high relative humidities. However, the maximum water layer thicknesses obtained for the dried leaves were much lower (8.8-17.9 m). We postulate that the cuticular membrane may behave as a ''valve'' between the inner and outer region of the leaf and that its permeability is controlled by the relative humidity of the air. This view may be a fundamental contribution to the representation of the mechanism of the dry deposition of gaseous compounds to the vegetation.