Plasma glow-discharge application is known as a technique to coat or modify the surfaces of various materials. In this study, the influence of oxygen rf-plasma treatment on surface and bulk properties of a biological polyester, poly(3-hydroxybutyrate-co-3-hydroxyvalerate), were studied by determining water content and water contact angle, and by using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The plasma-treated films absorbed more water than the untreated film, and the absorbance increased with the total power applied. The water contact angles decreased and O/C atomic ratio increased on treatment, indicating that the material became more hydrophilic due to increases in the oxygen-containing functional groups on the surface of the polymer. A direct relation could be observed when the O/C ratio was plotted against the total power applied (treatment duration X treatment power). SEM revealed a visual record of surface modification, the extent of which increased with increased total power. It was thus possible to alter the surface chemistry and relevant properties of the polymer film using oxygen plasma as a tool. (C) 2002 Wiley Periodicals, Inc.