Poly(ethylene terephthalate) was exposed to oxygen and carbon dioxide plasmas for different periods of time. The surface-modified samples were characterized by infrared spectroscopy, atomic force microscopy, and inverse gas-solid chromatography. The main difference between both types of plasma was connected to the time scale of degradation, which was much faster when using oxygen plasma. Aggregate globular features were produced by different treatments due to chain scission and further recombination of evolved products. Oxygenated functionalities were introduced in significant amounts after long exposure times to the oxygen plasma. As a consequence, the specific component of the surface free energy was clearly observed to increase after these long treatments.