X-ray photoelectron spectroscopy (XPS) was performed in situ on plasma-polymerized silica-like films that were deposited onto metal substrates. Relatively thick films (similar to8.0 nm) had spectra that were typical of bulk amorphous silicon dioxide (a-SiO2). When thinner films were analyzed (similar to2.4 nm), a Si(2p) peak emerged that was due to the formation of silicon suboxide at the interface. Changes in the metal and metal oxide peaks showed that oxidation of the substrates during plasma etching and deposition occurred. It was determined that during the initial stages of plasma deposition, metal atoms from the substrate migrated to the metal-oxide surface. This resulted in preferential oxidation of metal atoms with the formation of silicon suboxide at the film/metal interface. In addition, interfacial suboxide formation was shown to have a dependence upon the diffusivity of the metal substrate atoms through the surface oxide of the metal. As a result, more interfacial suboxide was observed to form for depositions on titanium substrates in comparison with depositions on aluminum substrates. A detailed analysis of the atomic species detected with in situ XPS enabled us to develop a model of the molecular structure at the a-SiO2/metal interface for plasma depositions on aluminum and titanium substrates. When the possible chemical reaction routes for film deposition were considered, the formation of primary Al-O-Si and Ti-O-Si bonds at the interface was proposed.