The change in the water contact angle for poly(ether ether ketone), polycarbonate and poly (methyl methacrylate) with treatment time in microwave oxygen plasma is discussed as well as the aging properties of the plasma treated polymers. XPS and the micro-Raman technique were employed to provide data on the extent of surface oxidation of polypropylene with treatment time. The microwave discharge creates a high density of free radicals on the plasma treated surface, enabling an uptake of air after sample exposure to ambient condition, and contributes to the polymer weight gain. Under the employed plasma conditions no micro-etching and substrate removal that could result in an increase of the polypropylene roughness were observed for oxygen plasma. The surface modification of polymer materials can be performed in low-cost microwave reactors in a cavity discharge configuration.