Plasma-polymerized hexamethyldisiloxane (HMDSO) films (similar to 800 Angstrom in thickness) were deposited onto aluminum substrates (6111-T4 alloy) in radio frequency (RF) and microwave (MW) powered reactors to be used as primers for structural adhesive bonding. Processing variables such as substrate pre-treatment, carrier gas, and him posttreatment were adjusted to produce films that had different structures and properties. The plasma polymerized films were characterized by reflection-absorption infrared spectroscopy (RAIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and ellipsometry. Films deposited using argon as the carrier gas were siloxane-like. When siloxane-like films were post-treated with an oxygen plasma, a silica-like surface layer was produced. Silica-like films were deposited using oxygen as the carrier gas. Plasma polymerized silica-like films were formed of spheroidal particles deposited next to each other and impinging upon one another. These spheres were thought to be islands formed by the growth of stable nuclei. Etching aluminum substrates in argon and argon/hydrogen plasmas before deposition of the primer removed adsorbed water from the surface, possibly creating a more stable oxide surface to which the silica-like primer could bond. The effect of etching silica-like films in an Ar plasma was to promote the condensation of adjacent silanol groups to form Si-O -Si bonds but with little change in the film thickness or the concentration of isolated silanol groups.