The adsorption of water on hydrophobic organized organic thin films, used as a proxy for atmospheric organic aerosols, was measured simultaneously with a molecularly controlled semiconductor resistor (MOCSER) and quartz crystal microbalance (QCM). Water adsorption was found to be reversible and dependent on relative humidity (RH). The MOCSER measurements show that the adsorption kinetics of the first water layer resembles Langmuir behavior. The QCM findings indicate that adsorption is proportional to the water vapor pressure and exceeds the amount equivalent to one monolayer. These results can be explained by the formation of small water clusters on imperfections or structural defects on the organic surface and that the water does not achieve complete surface coverage even at high relative humidity. This mechanism resembles previous observations of water adsorption to structural defects on inorganic crystals. It is also shown that water can penetrate through the organic surface and reach the inorganic surface, even with a closely packed organic coating. This paper presents applications of these findings to the properties of organic aerosols. ¿ 1999 American Geophysical Union