The influence of argon plasma on polycarbonate (PC) was studied in terms of structural changes, reaction mechanisms, and adhesion, In situ ultraviolet-visible ellipsometry reveals formation of a surface layer with a higher refractive index than the untreated polymer. The increase in the refractive index is attributed to polymer densification, which in turn is attributed to crosslinking. However a decrease in the average molecular weight is also observed, and two populations of macromolecules of different sizes are detected by light scattering measurements, revealing a competition between crosslinking and degradation. The reaction mechanisms are investigated using nuclear magnetic resonance and in situ infrared ellipsometry. Degradation is caused by carbonate bond breaking, whereas crosslinking seems to be related to a decrease in methyl groups. In addition, an increase in surface acidity is detected by contact angle measurements and is attributed to photo-Fries rearrangements producing phenolic groups. The adhesion of a 1-mu m-thick silica layer on PC is evaluated by the microscratch test, and its enhancement by argon plasma pretreatment is analyzed.