Interactions between a late Ar-O-2 post-discharge and thin films (similar to 1 - 2 mu m) of stearic acid (SA), a C-17 aliphatic chain with as end-group a carboxylic acid function, are studied. Thin films grown by evaporation are made of separated droplets and are efficiently etched by the post-discharge to get a clean surface after treatment. The dewetting of the droplets during the first minutes of the treatment leads first to a more homogeneous film in thickness which is etched until isolated islands appear and progressively shrink as a function of time. The etching of the SA occurs via C-C bonds scission when peroxide or hydroperoxide compounds are synthesized from radicals initiated by reaction with atomic oxygen from the post-discharge. A strong functionalization by OH groups grafted on the aliphatic skeleton of the SA is also observed. C = O formation is observed for longer treatment times possibly due to oxidation of OH grafted on the aliphatic chain or to a modification of the branching ratio of the C-C bond scission mechanism due a temperature rise. The acid functional group is likely not modified by the treatment.