Reflection-absorption infrared (RAIR) spectroscopy was performed in situ on plasma polymerized silica-like films that were deposited onto metal substrates. Relatively thick films (similar to8.0 nm) had infrared spectra that were typical of bulk amorphous silicon dioxide (a-SiO2). When thinner films were analyzed (similar to0.6 nm), other infrared bands emerged that were due to the formation of silicon suboxide at the interface. Infrared bands due to oxidation of the substrate during deposition were also observed. 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.