The diffusion process from water into and out of the bulk of an injection-molded bisphenol-A-polycarbonate: sample has been characterized by weighing. In the equilibrium state, the amount of water inside the polycarbonate depends, therefore, on the environmental humidity. Transferring the experiments into UHV showed that the amount of water could be determined by the time-integrated H2O-signal of a mass-spectrometer as depending on the storage rime in vacuum before measurement. After Ar-plasma etching; samples showed a reduced H2O-signal, which is the consequence of a hydrophobic barrier on the sample's surface created by the plasma. The thesis of a barrier was supported by XPS-measurements, which showed a decrease of the polar carboxylic group of the polycarbonate.