Emission spectroscopy was applied to observe the reaction process of poly (ethylene terephthalate) (PET) in an oxygen (O-2) plasma generated by a microwave discharge. As the PET was exposed in the O-2 plasma flow, light emitted from the PET surface was monitored. In the diagnosis measurement, several emission peaks assigned to the H-alpha, atomic line at 652 nm, H-beta at 486 nm, OH ((2)Sigma-->(2)Pi) transition near 244 - 343 nm and CO (b(3) Sigma --> (3)Sigma) near 283-370 nm were observed and measured at various discharge times. These results indicated that after the plasma etching, the PET sample was decomposed by the oxygen plasma reaction, and then, hydrogen abstraction and carbon oxidation processes. We also observed the time profile of oxygen atom, as the atom-emission intensity at 777 nm was monitored. As H-beta atomic and OH molecule lines appeared in the presence of PET, the O atom intensity was significantly reduced. In the surface analysis on Fourier transform infrared and x-ray photoelectron spectroscopy measurements, it was found that for the PET surface treated by O-2 plasma containing excited atomic oxygen species, ester bands were broken and carbonization formed on the PET surface. (C) 2004 American Vacuum Society.