We investigated the influence of plasma pretreatment in Ar and in an Ar-N-2 mixture on the adhesion of plasma-deposited silica layers to polypropylene (PP). In situ UV-visible ellipsometry measurements enabled us to detect cross-linking at the polymer surface, probably induced by UV radiation emitted by the plasmas. The cross-linking mechanism involves methyl group abstraction, as recorded by in situ IR ellipsometry. The adhesion of silica to PP, estimated from the micro-scratch test, is improved by both kinds of treatment. In the case of Ar, the improvement is attributed to cross-linking, and possibly to surface activation by radical formation. In addition, a significant nitrogen grafting is observed after Ar-N-2 treatment : amine and imine vibrational modes were detected using IR ellipsometry, and contact angle measurements show a significant increase in the electron-acceptor and especially electron-donor components of the surface free energy. Better adhesion after Ar-N, treatment than after simple Ar treatment may be attributed to C-N-Si bond formation or to acid-base interactions between the nitrogenated polymer surface and silica.