Poly(ethylene terephthalate) (PET) was treated with vacuum UV (VUV) (lambda =104.8 and 106.7 nm) photo-oxidation using a microwave (MW) plasma of Ar as the radiation source and the results were compared to treatment with O atoms produced from a remote MW plasma of a gaseous mixture of Ar-O-2 in the absence of radiation. X-ray photoelectron spectroscopy (XPS) detected a lower saturation level for the atomic % of O with VUV photo-oxidation (32 at%) than with O atom treatment (34 at% O). The C 1s curve fitting results for both treatment methods show a decrease in aromatic sp(2) carbons and an increase in the amount of carbon-oxygen bonding due to the formation of the carbonyl (C=O) and carbonate-like (O-(C=O)-O) moieties for VUV photo-oxidation; and C-O-C, C=O, and O-C=O moieties for O atom treatment, respectively. Advancing water contact angle measurements showed more of an increase in hydrophilicity with the O atom treated samples than VUV photo-oxidation. Atomic Force Microscopy images displayed smoother surfaces after VUV photo-oxidation while O atom treatment resulted in insignificant changes in surface roughness. Rinsing the treated surfaces with ethanol solvent partially decreased the at% O indicating the formation of a weak boundary during treatment.