Transparent, high-purity silica films have been synthesized on a polymeric material, that is, polyethylene terephtalate, by low-temperature plasma-enhanced chemical vapor deposition using tetramethoxysilane or tetramethylsilane mixed with oxygen. The effects of deposition conditions on the silica films' structure were investigated. Scanning electron microscope images clearly showed that the silica growth process depended on deposition conditions, including substrate surface conditions and the absence or presence of an oxygen fraction. Organosilane fragments are thought to have adsorbed selectively on energetically advantageous surfaces of the polymer substrates. Dome-shaped silica structures formed in this manner on the polymer surface in the early stages of the deposition. When no active oxygen species existed in the plasma, the boundaries between these domes remained in the deposited film even when deposition time was increased. On the other hand, in the presence of activated oxygen species in the plasma, these silica domes fused to each other and grew well with increased deposition time. Finally, densely-packed silica films were successfully formed on the polymer substrates. The silica films synthesized in this study were found to grow on the polymer substrates in an island-like homogeneous manner.