The effects of Ar, H-2/Ar, O-2/Ar, and H-2/O-2/Ar downstream microwave plasmas on the surface structure and properties of polyvinyl chloride films are described. Plasma-polymer surfaces interactions are monitored using in situ infrared reflection-absorption spectroscopy. The influence of downstream microwave plasma conditions (e.g., pressure, gas composition, irradition time) on polymer surfaces are studied, in either the direct or the indirect exposure configuration. Indirect downstream microwave H-2/O-2/Ar plasma exposures under appropriate conditions do not alter polymer surfaces to the detection limit of the infrared technique. Complementary results are provided by pressure changes in the plasma reactor system, water contact angle measurements, and surface morphology studies on polymer surfaces using atomic force microscopy. The results of similar plasma treatments on polyethylene surfaces are described briefly.