The "interphase" region between the deposited layer [e.g., plasma-enhanced chemically vapor deposited (PECVD) SiO2 or SIN] and the poly(ethylene terephthalate) (PET) substrate has been investigated and compared to physical vapor deposited (PVD) (electron beam evaporated) SiO2. Composition profiles determined by time-of-flight elastic recoil detection, electron microprobe analysis, and x-ray photoelectron spectroscopy all show an extended interphase region more than 50 nm in width, while the profile of the PVD SiO2 is narrower. However, since these analytical techniques are invasive and prone to artifacts, we have also examined ultrathin (about 10 and 20 nm) SiO2 and SiN PECVD layers on 50 nm spin-coated PET substrates by nondestructive infrared (IR) techniques. The IR spectra confirm that the thin PECVD deposits also comprise an organosilicon phase with Si-CHx bonds. We explain these observations in terms of a fragmentation/redeposition mechanism : During the earliest stage of PECVD, interaction between the plasma and the polymer surface produces volatile organic species, which intermix with the reagent gas feed, thus giving rise to the observed organosilicon-like deposit with gradually decreasing carbon content.