Plasma treatment can be used to modify the surface properties of polymers. In the present paper angle resolved ESCA studies were carried out to determine the influence of a H2O plasma on polysterene. Contrary to an O2 plasma we observed a thinner reaction layer. On the other hand the H2O plasma treatment induces higher concentrations of carbonate and ester linkages. An additional H2 plasma treatment demonstrated that the highly oxidized constituents were reduced more effectively. The plasma reactions were carried out in a reactor that was already attached to the ESCA spectrometer.