Plasma polymerization of hexamethyldisiloxane (HMDSO) and hexamethyldisilazane (HMDSZ) monomers has been carried out in audio- (2 kHz, 20 kHz), radio- (5 MHz), but principally in microwave (2.45 GHz) frequency discharges. Film growth kinetics on glass and aluminum substrates have been studied by thickness and gravimetric measurements; the combined data have led to precise determinations of the film densities. The density data, in turn, provide strong evidence for a hitherto unreported structural feature, namely, that films are composed of a dense solid phase, and a thin (∼ 500 Å) “oligomeric” surface layer. The solid component is about 70% denser than the equivalent “conventional” polymer, and has a marked inorganic character, as revealed clearly by infrared spectroscopy. IR analyses of polymers produced in lower frequency discharges tend to show narrow, well resolved absorption bands. In the case of 2.45 GHz films, on the other hand, bands corresponding to “inorganic” linkages tend to become broader and more intense, and structural features become smeared-out. This is indicative of extensive crosslinking and of the inorganic structural content already mentioned. These results indicate intense fragmentation of monomer molecules in the microwave plasma, as also substantiated by pressure-rise measurements. Radical species created by abstraction of hydrogen, methyl, or other groups are responsible for film formation and growth and for crosslinking reactions within the film.