Polyethylene terephthalate (PET) surfaces were treated with CH 4 /CF 4 plasmas and the surface structures as well as the dynamic behaviour of the modified surfaces after water immersion were studied using angledependent x-ray photoelectron spectroscopy (XPS). It was found that the fluorocarbon plasma treatment resulted in the formation of some fluorine-containing groups, which improve the surface hydrophobicity of the PET films. The extent of surface modification is dependent on the molar ratio of the CH 4 /CF 4 feed gases used in the plasma. The hydrophobicity of the modified PET films decreased after they were immersed in water, partly as a result of the 'turnover' or movement of the fluorine-containing groups from the surface to the bulk. From XPS analysis, it was found that the movement of the fluorine-containing groups depends on the modified surface structures. The turnover rates can be described by a decay constant k of surface dynamics; the larger the k value, the faster the movement of the fluorine-containing groups. Experiments showed that the degree of surface modification is higher and is maintained for a longer period for CH 4 /CF 4 mixed plasmas as compared to that treated by pure CF 4 plasma. The effect of the molar ratio of CH 4 /CF 4 feed gases and its relationship with the movement of the fluorine-containing surface groups were discussed.