The effects of oxygen plasma treatment and the subsequent air exposure on the surface composition and properties of bisphenol A polycarbonate (BPA-PC) were analysed by X-ray photoelectron spectroscopy (XPS), ellipsometry, static time-of-flight secondary ion mass spectrometry (ToF-SIMS) with principal component analysis (PCA) and nanoindentation using an atomic force microscope (AFM). PCA showed systematic changes in the film chemistry after short treatment times (0.1 s), with the main sites of attack being the carbonate and aromatic ring structure. On the basis of this multitechnique analysis, it was unambiguously determined that extended oxygen plasma treatment times resulted in the formation of low-molecular-weight material (LMWM) within the first 50 nm on the surface, and not in a cross-linked skin as has been proposed by other researchers. The study shows that controlled surface modification of BPA-PC polymers is possible, allowing surface oxygen incorporation without degradation of the polymer structure. This result is relevant for improved adhesion of coatings applied to BPA-PC polymers.