We have studied the influence of plasma treatment in various gases (Ar, NH3, N-2) on the surface modification of polycarbonate (PC) and on the adhesion of plasma-deposited silica layers to PC. Surface modification was investigated using in situ IR ellipsometry, X-ray photoelectron spectroscopy, and contact angle measurements interpreted in terms of electron-acceptor/donor groups. In addition, in situ UV-visible ellipsometry enabled crosslinking analysis. Due to UV photon emission, Ar plasma treatment induced crosslinking as well as photo-Fries rearrangements and related reactions, thus creating phenolic groups. These groups are acidic sites and are likely to react with the electronegative oxygen atoms of silica. Adhesion of silica to PC, as measured by the micro-scratch test, was enhanced by Ar plasma treatment. The improvement is attributed to the crosslinking of PC, on the one hand, and to acid-base interactions and/or covalent bonding between PC and silica, on the other hand. A further improvement in adhesion was achieved using N-2 plasma treatments (pure or Ar-diluted). These treatments also induced crosslinking and phenolic group formation and, in addition, nitrogen grafting at the surface. Adhesion enhancement from Ar to N-2 treatments is thus attributed to nitrogen-containing groups, which are likely to promote covalent bonding between silica and the treated polymer surface. In contrast, Ar-diluted NH3 plasma treatment following Ar treatment resulted in decreased adhesion, which is attributed to reduced acidity and low nitrogen grafting.