Interactions between a late Ar?O2 post-discharge and biphenyl (C6H5)2 are studied. Thin films grown by spin-coating are efficiently etched by the post-discharge to get a clean surface after treatment. Thin films are strongly preferentially oriented. But this orientation does not depend on the way the sample is pretreated. The pretreatment can create cracks and modify the etching rate. The etching of the biphenyl occurs by interaction with the singlet state of molecular oxygen O2(a1?g). [4?+?2] cycloaddition is assumed to be the main process leading to ring opening. Next, a large variety of compounds including alcohols, ketones, acids, and aldehydes are created. Atomic oxygen does not seem to play a significant role in the etching process but it functionalizes the biphenyl by creating alcohol groups.